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Diagnosing Bad U-Joints

JeepMaz

NAXJA Forum User
I have a '98 4x4 Classic with 4.0L and automatic trans. Within the last few months, I have noticed a mild clunk coming from the rear end sometimes (not always) when I accelerate from a stop. It feels like something slips just a bit before engaging. I also have a decent shimmy right around 60mph that is nonexistent at higher or lower speeds. Could this be bad U-Joints? If so, how else can I diagnose it? Roughly how much would this repair cost?

If not U-joints, any other suggestions? Tranny fluid is good. Haven't checked the rear diff oil or t-case.

Thanks.
 
Check the U-joints on the rear driveshaft. Hold the yoke with one hand and the shaft with the other, hold the yoke fast while trying to spin the driveshaft with the other hand. If the yoke and shaft can move independently, even if a little bit, the U-joint is shot. The rear shaft has two U-joints on it, check them both.

The clunk could also be a problem inside the rear. You'd have to pop the cover and take a look.

The shimmy could be tire balance, start by having a shop balance all 4 tires. Could also be a slipped belt, uneven wear, or other tire issue.
 
It's highly likely that the problem you described as "something feeling loose" is the slip yoke on the rear driveshaft. When I got my 96 it had about 45,000 miles on it and I noticed right after I got it home. I thought for sure something was wrong with either the trans or the rear end. I described it as feeling kinda sloppy or loose. What happens is that boot at the transfer case end becomes dry and it sticks...and clunks. When you stop or go the driveshaft will compress or lengthen.

Take an 8mm wrench and take off the 4 bolts on the u joint straps at the pinion end of the driveshaft. You'll probably have to take a big screwdriver and stick it somewhere between the u joint and the yoke so you pop the whole thing loose. Then you just pull the driveshaft to the rear of the vehicle and the entire thing (boot connected) comes off. But a bunch of grease down in the boot and then put it all back together. I know you're not supposed to re-use those u joint straps and bolts but I have. I believe the torque on the bolts is something like 5 ft lbs. Good luck.

-river
 
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