I have a 1978 f150 short-bed stepside 4wd with 4spd manual transmission. At approximately 40-45 mph i experience a horrible vibration coming from the back of the truck. Initially I thought it was a bearing. I replaced the bearing that was bad and the vibration is still there. I looked underneath the truck and saw a nasty leak coming from the back of the transfer case by the yoke. What would the problem be and could this be the cause of the vibration somehow?