1998 Ford F-150

I would probably start with the rear end. The gears inside it have an 'accelerate' side and a 'decelerate' side. If a pinion bearing is going bad, when you let off the gas, the pressure from the ring gear will push the pinion away from it. This could possible cause a vibration ONLY when you let off the gas. Try this. Drive it at the speed in question and let off the gas. If it starts vibrating, put it in neutral and see if that makes a difference. This will take the strain off of the driveshaft and possibly stop the vibration. If the vibration goes away when you do this, check the rear end. You might even check the U-joints as well just to be safe. They can cause some wierd problems when they start to go bad. Let me know how it goes and what you find. Good luck