There is a variety of things that could account for your rear end sag. I've never noticed shocks to make it sag to where it was visible (at least while the car's stationary). The steering damper is probably not going to help anything, as its the rear end in question. I would start with a new differential mount and subframe mounts. The parts for these jobs is not too much $, and it's where most of you issues probably lie. I would look toward the springs next.

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1989 300E 144K