rearend help

When does it clunk? Is it when you're taking off after a stop or is it more over bumps? Or is it when you launch it hard? Are there any vibrations at speed?
I have some major deflection in my control arm bushings that causes a clunk over bumps and the driveshaft to hit the floor when I launch it (soon to be R&R'd). If the car has that many miles and you are still running stock arms and bushings, its high time you change those out, they're likely all cracked and rotted. If the gears were worn bad enough to clunk, I would think that it would be accompanied by lots of other noises like whining and vibrations. Only other things to check would be to make sure the sway bar is in good shape and the bolts are tight, and the shock mounts are tight as well.

Deffinetly check your control arm bushings and replace as needed. While you're under there, its very unlikely to be the case unless the car has been run hard with drag slicks, but check to see it the torque boxes are cracked at all around the welds. I doubt that would be the case but it happens to be that, its a potentially dangerous situation that should be rectified sooner than later.