2002 T&C LWB rear vibration on the highway
 

The problem I am having is a vibration in the rear starting around 60 mph and up. I have new tires with about 1200 miles on them. I had a vibration in the rear at around 800 miles on the tires and one of the tires in the rear had a little bit of a hop to it so they replaced it no charge. Now its back and I took it in today the tire dealer balanced and switched sides with the tires and its still present. There is no noise coming from the rear and the shocks are in good shape. Would a bad brake rotor cause this or could a bearing be on the way out? The brakes only have about 20k on them and most driving is done in town at speeds of 25 - 35 mph