This will need to be diagnosed to know what needs fixing, but I have a very inexpensive possible fix. Try adding a bottle of Chevron Techron Concentrate Plus, this additive is added to a full tank of fuel, and it has the ability to clean the fuel tank level sensor and fix some of these gauge issues, all for about $10 - $15.00. I have recommended this to a few people in the last month and it has fixed 2 fuel gauge issues. Run the engine on a tank of fuel with this additive, and it has to be this exact type, and if the gauge doesn't start working, then a diagnosis of the problem will be needed.
senting united is in the gas tank, replace it and the fuel pump only cost 120.00 but to have it put in will cost about the same you need to let the ford place do it becaue the hose where you put gas in need to be right or it will come back on you evry time you put gas in.
The fuel gauge and fuel sending unit are very easy to test and in about 15 minutes you will know exactly what's wrong.
First thing you need to do is check all the fuses and make sure you are getting power to the gauge. If the other gauges are working, then you can safely assume power is coming into the cluster (red/yellow wire).Then check the black/yellow wire at the fuel tank connector and make sure you have a good ground.
If power and ground are good, then ground the yellow/white wire at the fuel tank connector. If the fuel gauge goes all the way to FULL, you have a bad sending unit. If it stays at Empty, you have a bad gauge or an open in the yellow/white wire.
Ford has a special tester for the Anti-Slosh module. But if you replace the fuel level sending unit and fuel gauge, and assuming the wiring is indeed good, then it's a pretty good bet the Anti-Slosh module is bad.