If you have Bilstein gas shocks they are pre-loaded with gas. If one of them has a leak it will certainly give a less upward force. (Lowering the car in that corner). If you didn't notice any difference between the two rear shocks when replacing the springs this theory will probably not be worth anything, but a cheap test is to check if itś a difference in resistance when you press down the rear fender on each side of the car. When lifting the car by the differential the difference in hight will be probably still be there, but not as much as it would be if the car was standing on ground.