I had the same problem as you. I have the Borla cat back exhaust. While it sounds great, quiet when going slow and opens up very nicely when snapping the throttle but I had a highway resonant drone which was annoying. The back panel has little to no insulation, the panel exterior has the cheap panel cover and the interior of the panel has that white polyester fill (like speaker boxes) which doesn't do much at all.
I first placed dynamat on the entire rear panel (pretty easy to do and actually left the rear seats in when I did it) and the rear doors. This made a dramatic effect in noise reduction, the wife and kids actually noticed and said the truck became much quieter. We recently took a trip to Virginia and the ride was so much quieter and enjoyable.
I recently installed a new sound system and added more dynamat to the rear floor board and one front door (ran out of time and too cold!) which made the truck even quieter, but no where as drastic as when I did just the rear panel. For $150-$300 for the dynamat it was a great investment. I was going to install luxury liner pro over the existing dynamat but I'm pretty happy the way it is but I think the front and especially the lower portion of the firewall may need both dynamat and luxury liner pro, which will be my next project in the spring.
