I have a 97 suburban k1500 i use to tow my 14ft tandem axle trailer with 2 big size quads on it. The Suburban will shake so bad from 65-75mph you would think something is going to break. The feeling seems to becoming from the rear of the truck. And you dont feel any vibration in steering. So i balanced the tires and had the u-joints replaced, thinking it was one of the two that was causing it. Well i was wrong because it never changed at all. But when i dont have the trailer on the truck rides smooth as it should be. Could the problem be the sag im getting when i hook up the trailer? it sags bad when i put the trailer on maybe causing drive line to be out of align?