The speedometer and gas guages do not work in my truck ('78). I have already replaced the speedometer cable, and that didn't help any. The needle never moves, and neither does the odometer. Could the gear in the transfer case need to be replaced? If so, how difficult is this? The gas guage reads all the way to the right (past full), until I get pretty low on gas. Then the needle bounces around. Would this be bad wiring, bad sending unit, or something else? Thanks in advance!!