I have a 2005 GMC 2500 duramax and I tow a new 303RLS reflection (just got it) You can see the pic in my profile (excuse the salt on the truck) and it is with my winter tires 245/16's on the truck....
My OEM shocks were replaced with Bilsteins, and they were quite expensive. The Bilsteins didn't last any longer than the Ford heavy duty shocks, however.
I found Monroe Gas Magnums at RockAuto.com for $29 each. I honestly cannot tell any difference in performance, and the price was just great.