Vehicles with automatic level control that ride harshly in the rear (no give in the suspension) may have one or both dampening actuators which have failed. It is commonly recommended the actuators be replaced in pairs.
Car Problem Reports
Mercedes-Benz 600SEL Bad Dampening Actuators Can Cause Rough Ride in Rear
Mercedes-Benz 600SEL Problem
Average Mileage: 87,413 mi (57,826 mi - 117,000 mi)