Actually, I'm pretty sure what you are describing is a common problem unrealted to the suspension. It is the rear window latch. Does it sound like someone didn't close a door all the way, but it is coming from back? Pretty common, mine does it, only on certain bumps, like sharp ones. Ones that always cause it are transitions in the joints in the freeway, like on elevated portions of the freeway. The have fix for it by putting a different bushing in on the latch. They require some play as an absorber I guess to keep the back window from dislodging on a good jolt.