I had this same problem. I spent a lot of money on new U-joints (all 6 replaced). The clunk was reduced. But, it wasn't until I replaced the diff mount and the moustache bar bushings that it went completely away. I even had the diff checked out. Point? Replace the mounts and bushings before you assume it's the diff itself.

How much are the mounts? Do I have to remove the diff to replace them?

I don't know how much the mounts are cause I usually get them for free.

they are easy to replace, I just unbolt the front diff mount from the crossmember, then loosen the rear nuts that hold the diff to the mustache bar (NOT ALL THE WAY) just enough to lower the front half of the diff enough to reach the two nuts on top of the diff mount.

I then replace the mount and work backwards from there.

All takes about a half hour with regular tools.

Dave.

Motorsport auto has them starting at $33.00 to $45.00

Hope that helps.

Excellent. I think I will go ahead and change the tranny and diff oil too.

As the original starter of this thread, I thought I'd let you all know how it finally worked out for me. As I couldn't find anything wrong under the car, one day I loosened the front diffy mount, sprayed WD40 all around, and tightened everything back up. Clunk is gone.

Go figure...