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My 87 had a noisey rear end, the spider gears were worn (rounded). It´s often hard to isolate the problem,, sound travels through the whole drive train. But a visual inspection and a little prying and shaking can tell you alot. Grab the pinion yoke, that piece sticking out of the differential and give it a good shake up ahd down any play and the shims are worn,the bearings are worn or the suicide nut is loose. Do the same thing with the axles, with the tires and drums removed, a little play up and down is OK (about a Millimeter) any more is a sign of worn wheel bearings (also if the seals are leaking). If you take the cover off of the differntial and stick a screw driver by the inside (carrier bearings) and there is any play up and down, it is a sign of worn bearings. Lastly grab the yoke and twist it left and right to see how much play there is between the pinion gear and the ring gear. There is always a little play, some of the Chiltons manuals give the allowable play which can be checked with a feeler gauge. I´ve done it enough times I can tell by the feel wether it´s right or wrong.
Other than worn spider gears, a loose pinion gear or wheel bearings I don´t worry about it to much. I just add a good bit of Moly gear additive to quiet it down a bit and drive it for another 40-60 thousand miles. Rear ends are pretty easy to find in a junk yard, and not that hard to change. Buy a new set of U-bolts and away you go.[addsig]