Driveline vibration!!! I need help!

It's in the driveline by what you describe. I think you'll find if you are the heavy footed type, that it will go away when you get to 75 or so, and probably come back arund 125. What exactly is the poinion angle? You said you checked it, but you didnt say what you had. I'm guessing the car has some rake to it... Super stock springs too? Also check the play in the slip yoke and trans tailshaft. There should be only about 1" of polished yoke showing with the car sitting on it's tires, with you sitting in it. Pinion angle or slop in the yoke can do what you describe. I'd also second the "have the rear tires dynamic balanced by a real tire shop. This would not be a Town Fair or Goodyear... A real tire/alignment shop...