I have an 89 RS, and every time I back up, stop suddenly, or just generally run over a bump, there is loud metallic thud. It's coming from underneath the passenger side, a bar that is connected from my differential to somewhere else. The rubber boot that keeps it from moving is missing, but I have no idea what it is called to buy a replacement. The bar runs almost the length of the car.

 

Sounds like you are describing the torque arm.... a long, stamped metal arm that extends from the front of the differential housing to the transmission tailshaft housing. It fits in a bracket, and the bracket contains a rubber bushing. If the bracket or the bushing fails, you will get a clunk.