what makes the rear shocks not stay firmly on the ground going over bumps , had rear passenger sidel

shock could be bad or the spring going bad look at the shock see if its covered in oil

The 140 Chassis S - Class was offered with a rear suspension "Anti-squat" system similar to the one used on ALL merc estate/wagon's. Basically when your setting off from the lights and accelerate hard, a valve connected to the anti-roll bar forces more fluid into the shockers to stiffen them up and prevent the rear end of the car dipping. More often than not the part that goes wrong is the control valve. It's a fixed unit under the car close to the rear anti-roll bar to which it's connected by a small rod, the trouble withthem is, they're exposed to the elements and tend to cease up especially if the cars been stood unused for some time. If your car has it (it was an option i think), it's a hydraulic system under massive pressure so any repairs should be done by a professional

If you have this hydraulic system the problem could also be the air cell. The rear shocks on such a system are simply pistons, the "shock absorption" is in the air cells. When they fail, the ride is very rough. To add to the earlier response, to check the control mechanism, you would need a lift--don't get under the car and operate it as the car will squat onto your chest and kill you--so have a experienced mechanic test the system.