Has a vibration I cannot find. At 65 plus MPH vibration felt in seat, go's away when I let off accelerator?

If the vibration is felt when the accelerator is pressed but stops when you let off, then this is a drive line vibration.

More than likely it is a driveshaft, so we need to not just balance the driveshaft but phase it and see if the vibration gets better or worse.

That means we need to rotate its position in how it is bolted to the rear end.

If you get the vibration to a lower level, then you can live with it or you will have to replace the drive shaft as it is likely bent or distorted.

https://www.2carpros.com/articles/how-to-remove-a-drive-shaft

I am attaching the process below but the way to confirm this is to use a vibration analyzer or a light system that shows if a drive shaft is out of round when installed in the vehicle.

Please let me know what questions you have, and we can go from there. Thanks