There are three basic types of driveshafts found in 4x4's. The most common type in newer vehicles is the tubular driveshaft with an attached slip yoke that slides into the rear of the transfer case. This driveshaft is found on Wranglers and Cherokees. The second type is a telescoping driveshaft. The shaft itself can collapse and extend along inner splines to accomodate suspension movement. This shaft can be found from the early model Jeeps to the last CJ-7. The third type is a lot less common on Jeeps. It's the much talked about Double Cardan or CV driveshaft. It uses two U-joints at the transfer case end, slip splines in the middle, and a single joint at the differential. Some front driveshafts of Cherokees and Grand Cherokees are of this type.

The first two driveshafts are designed to operate the same. Draw an imaginary line extending the output of the transfer case, and another line extending the input of the differential. The output shaft/yoke line from the transfer case and the input yoke line of the differential should be parallel. That is not to say they should point directly at each other, just parallel. This keeps both U-joints on the driveshaft operating at the same angles, and eliminates vibration.

The CV type is a little different. The same imaginary line extending from the differential yoke should point directly to the output of the transfer case. The U-joint at the rear differential would only operate at an angle when the suspension is compressed or extended.

So now you're thinking,"That's all fine and dandy, but why is my butt numb after a long freeway trip in my beloved Jeep?". Here are some of the most common causes and cures of vibration for each type of driveshaft.

If your Jeep, or other 4x4, is still stock and has over 50K miles on it, the most common cause of vibration is simply worn out U-joints. Most of the OEM U-joints are "lubed for life" and lack zerk fittings to lubricate them periodically. Consequently, the U-joints will quickly dry up, and their life is, typically, only a few years. The cure here is obvious. Replace the U-joints with a high quality, greasable joint, such as those from Dana/Spicer. Never replaced a U-joint? It's so easy you'll be wondering why people get paid to replace them. To replace the tubular driveshaft, simply unbolt the shaft from the differential and slide the shaft out of the transfer case (place a drip pan under the output of the transfer case as a small amount of fluid will drain out). The other types of driveshafts need to be unbolted from both the transfer case and differential yoke. With the driveshaft removed, use a pair of needle-nose pliers to squeeze and remove the U-joint retaining clips. Set an old socket on one end of the U-joint, and lightly use a hammer to drive out the opposite U-joint cup. The rest of the U-joint can then be removed by hand.

Installation requires a little more forethought. The concern is installing the U-joint in a position that the zerk fitting can be easily reached with a grease gun. The U-joint at the rear differential should be placed so the zerk fitting is pointing toward the front of the vehicle, and the U-joint at the transfer case should be placed so the zerk fitting is pointing toward the rear of the vehicle. Failure to do this usually means there is not enough clearance to use a grease gun to lube the joint. You'll also want to put both zerk fittings on the same side of the driveshaft, so you won't have to rotate the shaft to get to each U-joint.

If you've replaced your U-joints and the shaft still vibrates, or you're sure the U-joints aren't the cause of your grief, then it's time to dig a little deeper. With the tubular shaft, the slip yoke that slides into the transfer case needs to be examined for wear. The smooth surface on the outside of the slip yoke rides against a brass bearing in the tailcone of the transfer case. You can simply run your fingers up and down the yoke to check for wear, or use a set of dial calipers to measure the yoke width at several places. If there is any wear evident, then both the slip yoke and tailcone of the transfer case need to be replaced. The tailcone on the popular NP231 comes off with three bolts, and costs about 40 bucks at the dealer (don't forget a new output seal too).

The equivalent problem with the telescoping driveshaft is heavy wear in the slip splines. Under normal conditions (i.e. moderate engine power and regular lubrication), the slip splines will outlive the vehicle. However, since we're talking about Jeeps that are driven a whole lot longer than the average car, the splines are worth a look. With the driveshaft still in place and no load on it, check for play by moving the shaft up and down. The play should be very minimal. If there is a lot of play, it could be caused by either slip joint wear, or not enough spline contact after installing a suspension lift. Either way, you'll need a new driveshaft (about $150 bucks). Also, with this type of driveshaft, be sure that both the U-joints are in phase. If you were to separate the driveshaft, rotate one end 45 degrees, then reassemble the shaft, it would be out of phase. Both U-joints need to be at the same place in the rotation at the same time. Even if you were to rotate one end of the shaft 180 degrees so that the U-joints matched, you would ruin the factory driveshaft balancing. If you have to separate the driveshaft, mark it so you know how it goes back together.

So you've got new U-joints, there's minimal or no wear on the slip splines or slip yoke, and it still vibrates? Now you're beyond normal wear and tear; there's something else that has happened to your Jeep. Check the driveshaft for any dents or bends caused by rocks, stumps...whatever terrain your Jeep may have seen. Dents are easy to spot, bends aren't. If you suspect your driveshaft is bent, remove it, and have it balanced at a driveshaft shop. If it's bent, they'll know it, and be able to build a replacement on the spot. Even if you don't suspect the driveshaft is bent, it's not a bad idea at this point to have the driveshaft balanced, just to be sure. Odds are, the shop will do a better job of balancing than the factory did.

A more common cause of vibration comes from improper driveshaft operating angles after a suspension lift is installed. Since the suspension lift repositions the axles farther from the frame and transfer case, the operating angles change. Most of the manufacturers take this into account and and they will supply angled shims to place between the springs and axles to rotate the differential pinion to the proper angle. In some cases the manufacturers go to the extent of lowering the transfer case to further ease operating angles. However, due to manufacturing tolerances and varying vehicle weights, the shims or drop brackets may not cure the problem. You need to make sure these angles are correct. Eyeballing it will not cut it. There is a very inexpesive tool available from Craftsman which works very well for this job. It's called a "Universal Protractor" PN 39840 and it costs less than 10 bucks. There's no excuse not to have this item in your tool box. You can even order it on-line (http://www.sears.com/craftsman/). Using the tool to determine the operating angles is very easy.

Recall that in the first two types of driveshafts, the output yoke of the transfer case and the input yoke of the differential must be parallel. If the transfer case is pointing 5 degrees downward, the differential should be pointing 5 degrees upward. If not, you can use the shims or drop brackets mentioned earlier to match the angles.On this Cherokee with a 2" suspension lift, the transfer case is angled downward 4 degrees, and the rear pinion up 5 degrees. On this particular vehicle, the 1 degree difference does not cause any noticeable vibration at any speed.

You can also use the protractor to determine the U-joint operating range (driveshaft angle - pinion angle). In this case, the driveshaft sloped down 10 degrees, the pinion pointed up 5 degrees, meaning the U-joint sees 5 degrees of movement on either side of 0. The greater the operating range, the more movement the U-joint sees and the quicker it wears out. Anything greater than 10 degrees is seriously shortening U-joint life. Remember that in a CV type shaft, the rear pinion angle should equal the driveshaft angle.

Spending a few dollars fixing a driveshaft vibration may not be as fun as buying new chrome hood hinges, but it's money well spent. Besides, your Jeep can be pretty as can be sitting in your driveway, but if it runs and drives like junk, it might as well stay there.

On the way back from EJS2000, I started feeling a bad vibration coming from the underside of my Jeep. It felt like the tires were out of balance, as if one of the wheels had thrown its weights. Just to be on the safe side, I decided to crawl underneath the TJ anyway, and shake the driveshafts; good thing I did - the rear one wobbled like a fish out of water. If I had tried to make it to the next town (approximately 25-30 miles), I would likely have thrown the driveshaft entirely.

Since I happened to have a copy of the above article on my laptop with me, I set out to change my first U-joint right there, on the side of the highway.

With only 3 tools - a $4.95 ratchet set, needle-nose pliers, and a cheap grease-gun - I tore down my driveshaft, replaced the offending U-joint, and was on my way again in under 20 minutes.

It turned out that two of the cups on the bad U-joint had not been receiving oil, and most of the needle bearings had been ground to dust, along with part of the U-joint itself.

Thanks goes to Grant for taking the time to write this article in the first place (he's right, it IS easy), and to Les Schwab Tires of The Dalles, Oregon, for double-checking my work when I reached the nearest town...better safe than sorry.

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Dirt Road Magazine is sponsored by Turner 4WD Parts Co.