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 removed the 'Rancho' Ford FX4 OEM junk at approx 100,000 miles. The shocks had done their job I thought up until that point. When I compressesd them on the workbench, the fronts did not rebound. Others have found that same issue with the OEM 'Rancho' crapola after only 20-30k miles.