I've got a 1998 Jeep Cherokee with a pretty bad front driveshaft vibration.
When I remove the front driveshfat from the vehicle and do 60mph it doesn't vibrate. When the driveshaft is installed it vibrates.
I've replaced the U-joints in it, and even tried rotating the front axle for a better pinion angle (vehicle is slightly lifted) No change.
I've even gone to the wrecker to pick up a new shaft in case mine was bent, and no luck. Shaft continues to vibrate... I'm at a loss at what could be causing this.

You might have gotten a bad u-joint.

Drive line angle, bad universal joint, bend drive shaft, bad bearing in the front end, all can cause vibrations. Heck, even dirt stuck in one of the ujoints can cause it.