Most likely its the sender, but you can check it. There is an inline plug in the harness running from the fuel tank. It is located on the driver's side frame near the spare tire. Two wires for the pump and 2 for the sender. I'll have to look up the color codes, unless you have the wiring diagram. Check for corroded pins on the connector, then measure the resistance of the sender. With the tank empty it should be low, only a few ohms. With the tank full, it will be about 150-160 ohms. If it reads higher or open, the sender is faulty.
On mine, when I get down to a quarter of a tank, the sender rheostat hits a dead spot and opens up, causing the fuel gauge to climb to "Full".
Bob