560SEL rear suspension sagging to one side

[nhsu06]

Have a 560SEL that's sagging on the driver's side. When I bought the car last month, the rear suspension was extremely rigid (giving a harsh ride), and the car was sitting lower on the rear driver's side than on the rear passenger side.

> Just had the nitrogen accumulators replaced, which has fixed the rigidity problem and harsh ride
> The car still sits lower on the rear driver's side wheel
> When given a downward push, the rear driver's side bounces a lot more than the rear passenger side.

What are possible problems? Is one of the struts leaking? Or is it some other part of the hydraulic system (the level controller, pump, etc.)?

Thanks for your help!

-N