what about a loose flywheel??????
stuffed unis?

if it's when your coasting, it sounds like theres some slack somewhere, when you get on the gas the slack gets taken up.

If there is no vibration when under load at high speeds, why on earth do people think that its the angle?? the angle is still present when under load?

It sounds like it may be to do with the gerabox flywheel because your problem occurs when you change gears......