Start with the very easiest fix/diagnoses first
First, remember that our rear ends are over 40 years old. And all that time, most previous owners dont perform even the basic maintenance on them., let alone fix a prob that might lie in the rear end itself. It can get a little expensive if you need a rebuild
I recommend the following, in order \
1. Change out ALL the u joints: Front and rear; Its a a cheap job, and anyone can do it: Chances are your joints have never been changed. This might take the clunk out. Wore out u joints typically sound like that.
2. Check out all transmission mounts and engine mounts. Ensure they are not worn or loose, allowing tourque to flex the drive train to a point were you get a clunk
3. If you do the above, and I really recommend that you do, to rule out a wore out dif. If you do still suspect a rear end prob, you need to have it checked out by a pro
