This type of problem can drive you nuts trying to find. It can be as simple as an out of round tire or a bent wheel to a torque converter that is out of balance or off center to the crank centerline to an out of balance engine rotating assembly. Use Tech's troubleshooting sequence and advice given in previous posts and you will find the problem. The u-joint "out of phase" can easily be overlooked. Letting off the throttle can unload a lot of things from pinion and bearings, to u-joints, to unlocking the torque converter if u have this type converter. Keep us updated.