The most likely cause is the Coil Springs. The cost of the springs for about $150 for the pair you would also need to get the coil spring stabilizers not sure on the cost but could be another $50 - $75.
The labor could be as much as $800 and that will depend on labor rate in your area.
May, 12, 2011 AT 1:24 AM
The most likely cause is that the car was equipped with an automatic leveling system (air shocks with lines connected to an electric pump in the rear). The air shocks probably leak because of deteriorated rubber. The cheapest way to fix it is to get another rear end assy' (without automatic leveling) from a "you pull it" salvage yard and bolt it under your car.