RP199,
The only one that might be worth checking into is Bently's Auto (dieselpipes.com). They are in Coral Springs. Ask for Bently, he knows and has many contacts.
I have been wanting to order a 04 2500HD Crew Cab with D/A and will be adding a lift to it. Everyone I have talked to says these are the ones to do the work. All the dealers seem a bit clueless on the Duramax. Soon as more info on the new LLY engine comes out I will be ordering a new truck.