I think a close check out of the entire rear suspension is in order.... some measuing and checking the shocks for leaks should be part of this. Properly tighten all the bolts and make sure none of the mounts are cracked or bent.... Measure both coilovers at the installed height, they should be the same---if the shocks are good and the springs have the same tension. I would be for sure checking things over closely before I went for a drive in the car!!!!!