Either the gauge or the sender.
If you can reach it, pop the sender wire off the top of the tank and ground it. Key on, the gauge should show full. If it doesn't you have an open wire or bad gauge.
If it does you have a bad sender unit in the tank.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.