how to quiet down 3 inch exhaust

2 1/2" may not be your answer, I have a 440 with 2 1/2" x pipe, dynomax welded ultaflows, tail pipes out to the bumper, and I have a resonance from idle to about 2000 RPM, unless decellerating. I also have Eastwoods equivilent of Dynamat.
Before I put the Dynamat and the carpet back in, it was almost intollerable, now it's just annoying. I put the Dynamat all the way back under the rear seat where the exhaust mounts, it didn't help all that much.
When I'm over 2000ish RPM or decellerating, it's as quiet as a new car. I'm thinking of using the cheap glasspacks on the tailpipe as resonators like several have already suggested, but I have more important issues to deal with first.
It would be comforting for someone to post: "My car had a bad resonance and then I added glasspacks at the rear and now it sounds great, and doesn't hinder performance at all!"