Minivan rattles at 65mph in rear... what might be causing this?

Anyhow, I've noticed that lately the minivan shakes, rattles, and rolls towards the rear of the van at speeds over 65mph. Many people have suggested that the Sienna rattles due to poor suspension in the back. If so, what is the best place (I'm on a budget) to get this fixed so that the van doesn't shake in the rear at higher speeds. Thanks for any suggestions!