1993 Chevy Truck Vibriation

Have a 1993 Chevy K1500, When cruising at highway speeds there will be a vibration felt which feels like it is coming from the rear, if let off the gas the vibration goes away or if you step on the gas as to pass it will go away also. It only starts this if you slightly press the gas, there is no vibration when idle, and it will start at different speeds. Have replace the U-joints.