Interior Resonance

Does anyone have any experience in blocking some of the interior resonance in a Suburban? I installed an aftermarket exhaust system to increase power and fuel economy, but with the passenger compartment going all the way to the back of the vehicle we get a lot of interior resonance. I would like to keep the power gains etc., but was wondering about using some sort of insulation in the floors and rear interior panels.

Here is a suburban example:

http://www.b-quiet.com/suburban.html