2004 GMC Safari Vibration

My 2004 Safari vibrates terrible when above 50mph but not when accelerating, it's when you let go of the gas, it'll vibrate real bad. When it's above 60mph, It will vibrate all the time, because you can't accelerate fast enough to compensate. Although I've never gone fast enough, the faster above 50 you go, the worse it gets. It'll shake your teeth out and you can't see out of the mirrors. Also, when from 15-10mph, it kind of clunks in the rear and is real loud. Any ideas? Thank you, Justin.