Vibration

I have a vibration on the right rear rows of seats on my chevy venture. I know it has nothing to do with the wheel or tire because they have been rotated. It has also been like this since I bought the van and when I tell the chevy dealer they say they do not feel a vibration. What should I check on my van to determine the source of the problem. Thanks

Your rear tires could be out of balance. This would not be immediatly noticeable to you as a driver in the front seat. I can see the headrests on my second row captains vibrating while I'm driving, but no shake. The rear wheels are soo far back, you just don't feel it up front. It could also be a broken belt in one of the tires on the back, but the balance issue is probably more closely related. They do not balance the tires when they rotate them (unless you specifically ask and pay for it)
Hope this helps!