We have replace the front driveshaft four times.

It's either the wrong length or the wrong angle. The most common cause is incorrect ride height caused by weak springs or a lift kit.

If the length is wrong it can cause the u-joints to bind and set up a vibration, but you should feel that at much lower speeds. To only occur at higher speeds suggests the u-joint angle is not as it was carefully designed in. That is due to a change in ride height. Any tire and alignment shop will have a small book that shows every vehicle, where to take the measurements, and what they should be. If yours are okay, also look at the engine and transmission mounts to see if one has collapsed. That can also change the drive shaft angle.