U-joints usually cause vibrations under acceleration or deceleration, not at specific speeds. Wheels out of balance typically cause speed-specific vibrations.
Firmly set the parking brake and put the transmission in gear (manual trans) or in Park(automatic). Block one rear wheel and raise the other side of the vehicle. Slide underneath and grasp the driveshaft. Shake it back and forth and twist it. There should be no play between transmission output shaft yoke and driveshaft and the rear driveshaft yoke and the rear differential input yoke.