I have what I think is a similar problem. This is a 1994 S500, and the rear of the car sits rather low. I could live with that, but recently I am starting to feel a lot of "springyness" from the rear. There seems to be a definite lack of shock dampening.
Would this be a shock problem, or an accumulator problem. The car has 91k miles on it. And out of curiosity, how much would those parts be, and is it feasible for a DIY repair??