Found a topic on the 'open roads forum' that matches my situation. Between 40-50 MPH the truck vibrates (maybe a better term is 'kina feels likes it is gently hopping'. Only does in within that MPH range. Folks on that forum say they found different solutions. 1) tires, 2) U-joints and 3) Driveshaft balance. Anyone else heard of this? Any suggestions. Thanks.