The rubber bushings are your problem. I just replaced the bushing in my 230SL which had the same "problem" (sagging rear end). The bushing come in three heights and I chose the tallest. Visually, the difference in height of the bushings with weight on the springs was about 1/2 inch. Not only did it fix the appearance but it also corrected the rear end chamber. I got my bushings from Star Quality; $30 for uppers and $10 or so for the lower pads. Replacing them can be tricky if you don't have a spring compressor. Remove the trailing arm from the front, swing it down to remove the spring.

Good Luck

Jim Villers
190SL, 230SL 5-speed