I believe a common problem is that the ground wire for the sending unit rots away. There's really no way to get to it without dropping the tank.
It would probably be worth it to change the sending unit, but unless there's a leak or something else wrong with the tank I don't see the point in changing that... when I did mine, the tubing that feeds the fuel lines was also on the verge of falling apart, so I just replaced the entire unit.
Here is a link to also help diagnose what may not be working.. scroll down to the "Fuel and Temperature Gauges" section..
Jeep CJ Gauge & Sender Diagnostics