When I'm doing 45mph at 1400rpm the whole truck starts to shake violently almost hopping if you will. You can see the cab an box shaking. I've took off the tires and ran it up to speed that took like 75% of the vibration but some still stayed. Yes I've checked u joints and front end is tight also I took out my front drive shaft. My truck is lifted (1994 ext cab 4x4 318) I'm not to sure where to check next