[jku72]

This is a classic example of a driveshaft imbalance, but sometimes the causes and symptoms are harder to pinpoint.
The wah wah wah is usually caused by a harmonic imbalance. My truck does it, but not with your severity, and my problem is a worn rubber cushion in the center bearing housing. That wah wah noise has also been traced to flywheels being loose, or even the tranny bolts loose. The engine and trans are too heavy to detect loosness by moving, so try a couple of the bolts. tighten the converter to flywheel bolts, if you can.