I'm getting a harmonic vibration at approx 70 mph, more obvious under a load. Truck started doing it on a trip 1/2 way. Driveshaft has new ujoints and was re-balanced. No other changes. Seems to be a surging vibration, felt clearly though the floor. Can a worn tranmission cause this? (only has 65K on the trans - mostly highway miles) Truck is a 99 2500 4X4 24 valve. A/C on/off didn't change the vibration. Feels more like driveline

 

Putting it in neutral while at the speed the vibration happens will let you know if its engine or drivetrain related. A/C will have nothing to do with that though. Who installed those new u-joints and how long ago? It clearly sounds like u-joints or a driveshaft problem.