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....
I put a set of Bil's 5100's on our 2006 Dmax last Sunday. The factory shocks were worn out, which was to be expected. 114K miles. I haven't had a real chance to road test them, but the ride does seem to be much improved.
I got mine on Amazon, they beat the local parts store by over a hundred bucks for a set of four.
Took two hours to do the change over, but I wasn't in a hurry either.