http://www.maxcooper.com/rx7/how-to/...lls/index.html This is a great write up on how to change out your pillowball bushings to get rid of the clunk in your rearend.
Rearend clunk is normally caused by pillowball bushings going bad. You can get them from the Mazda dealer, part number is FD01-26-220. Sometimes the rear differential bushings go bad as well causing the driveline to feel sloppy. You can buy these from numerous other places/vendors. Feel free to chime in with your experience.