What Walker said. Civilian jeep has a great troubleshooting logic for all the gauges. Read up on it and get busy with your multimeter checking voltages and resistance on the gauge and sender.
WORTH SAYING: Check that your gauge cluster and dash have a good ground, preferably straight to the battery. Also, I ran the tank sending unit ground straight to the rear crossmember with an crimp terminal. I used "oxguard" anti-oxidation on this connections.