Since I purchased this truck in September I have had a vibration in the Drivetrain. It starts at about 30mph and is especially noticeable when accelerating or decelerating under load. Shifting to neutral, or driving not under any load, while in motion removes the vibration. It does not make any noise and gets no worse with speed. My first thought (and cheapest) was u-joints but they appear to be tight. My other thought was the salvage rear axle that was installed prior to my purchase. Any thoughts?