But...it turned out that we didnt have any upper control arm bushing .The cheap rubber ones get hard and crack,when I looked the only thing on the was the metal tube that the bolts goes thou.You could imagen the rear moving around.
1) Check the torque boxes for tearing (if you drive the car really hard)
2) Torn sheet metal where the UCA's attach to the body (remove your back seat to look at this area)
3) Could just be a bad shock
4) Bad bushings (already mentioned)
5) Check your bumper supports for rust (try to move body panels around)
6) Check your trans mount and floor pan areas for other tears...
These are the typical areas where the Fox mustang has problems