Try bouncing the rear bumper -- if it is very stiff or the rear end doesn't move at all, just the tires bulge out, your accumulators are shot. They are charged with high pressure nitrogen that eventually leaks out and causes the spring effect to go away.
Eventually, the rear suspension will lock up, with only the tires providing spring effect. Will bounce the h....l out of the car, tossing the rear end around on bumps.
These are fairly easy to replace -- you must fold the rear seats down and remove the trim panel between it the rear cargo compartment or third seat -- screws in about the middle, recessed into the carpet and some at the third seat hinge. Don't bend the panel!
From underneath, remove the hydraulic lines to the accumulators (use line wrenches, please!), then the three bolts holding them down. You will lose about a quart of hudraulic fluid. Messy, so have some pans ready to catch it.
Remove old accumulators out the top, then drop new ones in place -- don't tighten the hold down bolts yet.
Replace all the hydraulic lines -- thread the fittings in by hand, be carefull, they can be somewhat difficult to get started correctly. When the are all in and hand tightened, tighten the accumulator bolts and then finish tightening the hydraulic lines.
Takes less than an hour, vast improvement in the ride!
Use only the MB hydraulic fluid, and change the fluid if it is black, remember to put in a new filter.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!