First, you'll need to determine if this vehicle has shocks or struts. If shocks, having them replaced should correct the problem. If not, this condition could lie with the springs but that's unlikely since springs generally last the life of the vehicle. If this vehicle has struts, you can either have the entire strut replaced or, in many cases, simply have the strut cartridges replaced. Call a local dealer's service department and ask if the struts of the make/model in question have replaceable cartridges. To confirm worn or faulty shocks/strut cartridges do the following -- step down on the front and rear bumpers several times to get it bouncing and let off. If it continues to bounce more than once, the shocks/strut cartridges are worn/faulty. Take care of this problem very soon because, with each "thump", certain rear end components are becoming more damaged (most notably the "bushings")