Where are you feeling the vibration in the car? Is it in the steering wheel? If so, I would check balance on front tires and check the spindles. If you are feeling it in the seat it could be a few things. If you are certain on the rears being balanced, I would check the drive shaft and rear end like mentioned. As silly as it sounds, have you checked the heim joints? The other thought that comes to mind is checking the front uprights. The motor is also another good source for vibration if you think it is coming thru the chassis.