Waking up this old post. I found and resolvbed the issue. The front yoke spline and spline on the end of the drive shaft are badly worn. As you can see in the picture, the spline is worn to about half it's oroginal width. Bottom spline is the replacement driveshaft. Apparently under load it was hold the drive shaft in place. As soon as I would push in the clutch and remove any load on the drive shaft it was allowing it to move around and spin out of balance.
