BC52,

The vibration in the seat sure would indicate something in the rear of the truck.

The rear tires would be the obvious first choice, and easy to troubleshoot...put them on the front.

My money is on the drive shaft. Many times the urge to go the 'homemade' route is too strong, and whoever built the truck had his friend with the big lathe do the work.

If you think this is what it might be, you can use a couple of large hose clamps and do a "okie" balance job on the drive shaft yourself.

I can explain the balancing deal if you think it might help..or maybe you know it already.

Stuart