'97 E150 Conversion Van
 

I have an issue with my van that I cannot seem to get a hold on. I have a vibration that occurs whenever I go though about 68-74 MPH. Needless to say, this is the speed that I do most of my driving (lol). That's the only time it occurs - otherwise, the van rides pretty smooth. The vibration is in the seats. I don't feel any vibration in the steering wheel. I have done the following: Replaced the inner and outer tie rods on both sides (they were in pretty bad shape). Also, the front tires were replaced and balanced. I have also balanced the rear wheels. None of this made any difference in the symptoms - the vibration is exactly the same as before. I put the rear end up on jacks - leaving the wheels on - and ran the speedometer up through the range of 60-80 mph and it was smooth as silk - no vibration whatsoever! I'm sorta stumped at this point. Anyone have any suggestions about what to do next to find the source of this vibration?