If you have no problem with getting a little grease on your hands (you can use gloves, by the way), crawl under the van and try to yank the propeller shaft around to check if it is loose. Do the same with the exhaust pipe. It could also be the transmission supports, those rubber blocks that support the transmission underneath.

Getting to know your vehicle can really help save some money, especially since you wont be easily fooled into having a job done unecessarily, either by honest but ill-informed mechanics, or by scam artists pretending to be mechanics.