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....
Yeah, they won't change the ride height but are really helpful with ride comfort. Get the 5100's. You will be very happy especially if you're still on the OEM shocks. Check online for best pricing and make sure you get the correct shocks for you exact truck including any wheel changes over OEM. I put them on my 2007 classic Duramax. I also added air bags so I can adjust ride height if needed.