I have owned a 2007 GMC Sierra Classic, w/Duramax/Allison combo, 4WD, Crew Cab, Long Bed, for nearly 2 years. While towing my 5th wheel there is a vibration in the power train--most noticeable between 55-65 MPH. For nearly 2 years, the local dealer has been attempting to locate the problem, but has been unable to do so. Attempts to correct the problem have ranged from installing pinion angle shims, new driveshaft, etc. The combined GVWR for the truck and 5th wheel is 17,960 lbs, which is well below the rating for the truck. Has anyone with a comparable truck experienced a similar problem?