I have an 06 ExtCab ShortBed 2WD with the Duramax. I am using an MBRP Catback, been pretty pleased with it. Not a whole lot louder than stock, despite being bigger (4" vs 3.5") and the muffler is, well, let's just say a baseball would fall right thru it!
Picked it up lightly used off Craigslist, "cheap" because it was originally set up with an aftermarket exhaust brake on a crewcab truck (i.e. longer and hacked a bit right at the front). Was all prepared to have to fabricate things to make it work, ended up because my truck is shorter the "fab work" consisted of just shortening the front pipe (i.e. few minutes with a cutoff wheel) and bolting it it!
The MBRP has help up well (it is stainless) and looks great. Not going to claim miracles on either HP or MPG, but it didn't hurt on either.