2000 Pontiac Montana strong & loud shaking + vibrating.

Hi, our family van a Pontiac Montana V6 has for the past few weeks been shaking and vibrating to such a high level its almost as if we have a 22' sub in our car contantly on but while I have been trying to pinpoint the problem I have noticed when the I increase the RPM level over a 1000 the vibrating stops. Do you have any pointers on what the problem could be and how to fix it. Could I just increase the idle and if so how would I do that. Thanks!