Is it a rolling vibration or engine vibration?
While above 45mph if you push in the clutch and let engine RPMs go down does the vibration go away?
If not then it is a rolling vibration.
Could just be one of the wheels has lost a balancing weight, check each wheel inside and out for a "clean" spot where a weight may have been.
Block the wheels and put transmission in neutral, get under truck and try to move drive line manually if you feel it move try to locate a bad u-joint, it is usually fairly obvious, there is some play in the differential, that is normal.