If you have performed the test, and proved its in the rear,You have to decide if its worth it to pursue it.Those diffs will run with sloppy side gears behind slantys for thousands of miles.
But I guess it remains for an inspection to prove its side gears.Or R&P. Or crosspin.
The washers can be done in the driveway. The R&P is a lot harder to do lying on your back, plus requires some skills and tools. But an inspection is do-able.
So block the front wheels and get the back up,safely, to a comfortable working height.Slide a 4qt pan under the diff and pop the cover.Let the oil drain until it sorta stops dripping.Wipe most of the rest out, so as you dont get that stinky oil all over yourself.
Allright, lets find the clunk.Grab the driveshaft or the pinion yoke,with one hand, and the ring gear in the other.Immobilize the pinion and rock the RG back and forth.This is called checking the RG backlash. It ought to fall around .005, which is just perceptable with gearoil on it. A well-used unit, might have more. I would imagine some of mine have run double that.So if yours is less than .010, call it good.
Next, check the crosspin. On cars that have high-mileage or have been doing peg-leg burn outs, the crosspin tends to egg-out the case hole around it. If the crosspin is loose in the case, then the case is toast.
Next, rotate the case until the spiders are visible in the large window, and one spider is at the top.Grab a screwdriver and lift that spider up as high as it goes, then let it drop.If you more than about .030, you have found the clunk.You will need to remove the axles,extract that case, disassemble it, and shim the sidegears to minimize that clearance. While its apart check the crosspin to spider inside clearance.I couldnt find a spec for that. Im guessing if the pin is in good shape,and its not sloppy loose, just reuse it.Also check all the gear teeth to ensure they are re-useable.
If during shimming the spiders climb up too high, and get out of the normal wear area, they may need to get shimmed first.There needs to be some clearance in there when you are finished.I dont think theres any good way to measure clearance on well-used parts, but the guts need to turn freely.
Thats about it.Make sure the case spacers go back where they came from.Recheck the RG backlash.
