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....
Enjoy your new rig! Shocks are one of those things that wear over time so we do not realize that they are no longer functioning. If your current shocks are more than 3 years or 50K miles replace them, Bills are great shocks, and as stated will improve ride and handling! Good Luck!