Vibration under load

I have a severe vibration that started as faint and somewhat annoying, showing up mostly when the engine was under load (long hills, heavy acceleration). It's progressed to the point where it's very noticeable, still mostly under load. I can feel it in the dash and see the vibration (fuzzy mirrrors). I don't see or feel this when the car is stationary with the engine running. Any thoughts?

Hello;
Just a sugjestion. check your universal joints on your driveshaft. sounds like this would be a good place to start,with the vibrations under a load.does it do it when your coasting or slowing down? let us know what you find
Don

It's mostly noticeable under load, goes away when you take your foot off the accelerator. I was thinking maybe the blower belt tensioner pulley or harmonic balancer; the weird thing is you can rev the motor while sitting still without any noticable vibration.