Van shakes when accelerating?

(GMC panel Van) The problem started when I pulled up to a light and came to a complete stop, when the light turned green I tried to accelerate but pressing down the pedal made the whole van shake pretty violently and it barely accelerated. I tried letting it coast for a while and pulling over to check the transmission fluid and some other fluids everything was normal,, I did notice one other thing when idling in park pressing the accelerator does not make the van shake, it just revs the engine, it only shakes in the drive position. I left the car alone until today (about 6 days) so it wouldn't break or get worse. When I went to move my car for street sweeping this morning it was wobbling (that's really the only word I can think of) switching from park to reverse to drive made the van lurch a little bit (even with my foot on the break) and as it moved (I didn't press the accelerator just let it drift) it was wobbling side to side a bit .... whats goin on any ideas?