I have a 7.5" Fabtech lift on my truck with a 5" block in the rear. Before the lift I could drive it at any speed without the vibration. Since the lift and the 4.56 gears I have a vibration at about 80 mph. I've had the gears replaced once because of a faulty ring and it still does it. Is this going to be the norm or is it something that can be corrected? Maybe the u-joint or out of balance driveline? Any thoughts?
