When I bought my '06, it already had one of those mini straight-thru mufflers, maybe 6" diameter, 18" long. I cut it out and went to our local big rig store and bought a huge one ( roughly 30" x 12" ) 4" inside diameter all the way through, with three expansion chambers built in. It is probably as free flowing as a straight pipe, as quiet as stock, just deeper.
I was able to hook it up with just minor fabrication and I'm very happy with how it sounds. I like my truck quiet, yet deep. Also, I still have my catalytic converter in place.