Here some things to know on the self leveling suspension. The rear end is supported partially by the springs and partially by the rear struts.
The strut pressure is controlled by the the level-control unit which is physically located near and attached to the rear sway bar. Under no-load and low-load the level control unit maintains the struts at a nominal pressure to (1) partially support the rear end and (2) allow normal dampening within the struts. It's important to realize that each strut is connected to a nitrogen filled accumulater "ball"; the hydraulic fluid is seperated from the nitrogen by a flexible membrane and it's the movement of the membrane that provides the "shock absorption". But, fluid dampening occurs inside the strut similar to any other "shock".
Under heavier loads the level control unit lever is deflected and increases the system pressure whic counteracts the load, raising the rear end. Dampening and shock absorption still function as I just described. Once the load is removed, the level control unit is supposed to bleed the pressure down to the nominal system pressure which lowers the vehicle to normal height.
There are a lot of thing to fail. If the acculumators fail (ruptured membrane) then there is no Bounce to the car (a very rough ride) but the system pressure will be maintained and the ride height should be normal. If the the level controller fails, system pressure can be too high, too low or it won't adjust for a load. System pressure can be checked with a high pressure gauge (power steering type) and level control function can be checked by moving the level control lever to check its operation.
Struts typically give no problems for about 150,000 miles so are normally not a problem. Since they provide the dampening the "push down on the fender" test will usually give you a warm and fuzzy on their operation. AND they and very expensive!!!
I would have it checked but, if the car is too bouncy it's probably the struts. Try comparing to another s-class with the self-leveling rear; maybe it's suppose to be as bouncy as it feels to you
Hope that helps,