Are they still the original rear shocks? If so, forget the bumper test, they are shot at over 100k miles. Also, how are your tires / what kind are they? Are they inflated correctly? All can play a part in the ride.

But, I think you may have hit the nail on the head about a "truck" style SUV. Old MLs are body on frame, not unit-body. Solid as a rock, and ride like one too!