I just picked up a used '94 Limited 6cyl last week. It's in really good shape, other than a whining sound coming from the rear end.
The sound is constant, not pulsating. It can't really be heard until about 40 MPH. From there, the pitch of the sound increases with speed. It's not a grinding or mechanically violent sound; it's a constant tone. The sound is loudest when cruising. It goes away when coasting with my foot off the gas, and gets quieter under acceleration or going up a hill. It's definitely coming from the rear axle area, not transfercase or tranny.
I drained the fluid fromt the rear diff and put in some synthetic 75W90, but the sound is unchanged. The old fluid looked pretty good, though there was a bit of fine gold colored flake in it. The diff gears didn't look worn at all.
I figure the problem must be in some rotating part in the rear end. I'm loathe to take this to a Jeep dealership, as I'm sure they'd want to charge me $79384754 to rebuild my entire rear end.
I've seen talk of a pinion bearing...is this the input bearing to the rear diff? Could driveshaft u-joints be making this noise? There are bearings in the diff for each axle, right? What's on the wheel end of the axle - a sealed hub assembly, or a replaceable bearing? Anything else I should look at?
Any input/suggestions greatly appreciated.