GROUND...GROUND...GROUND. Check all the ground straps. Bed to frame, cab to frame (located just under the rocker panel),front clip to the frame(just ahead of the right front wheel) Motor to the frame,cab.There is a small ground strap from the inst. panel printed circuit to the dash. Sounds to me like you are acquiring your ground through the transmission/drive shaft/rear end/ springs/to the frame.I had a truck one time (auto) that the lights would only burn when it shifted to hi-gear.Good luck...SCRAP
