I dug in and learned a fair bit about my 2000 TJ gas gauge problems and was able to fix mine today for no cost which is of course, very cool.
My fuel gauge has been reading empty since I bought it a few months ago. Also the low fuel light never came one.
I was thinking I'd have to drop the tank & replace the fuel pump/sending unit but that is expensive so looked for other tests before I started on that.
I found a cool trick with my model of TJ, probably applies to other TJ's as well. You can test all the gauges following this very simple "Actuator Test" procedure:
1) Press and hold the speedometer trip reset button
2) Put in key and turn to ON (don't turn it to start! - only to ON position)
3) release the trip reset button
This makes the all the lights flash and the all the gauges do a self test. Particularly interesting to me is that my Fuel Gauge sprang to life and went to 1/2 tank, then full tank then back to 1/2 tank and then back to empty. More details of this test are here on page 45 & 46 of 434: 1999 Jeep TJ Wrangler Service Manual - 08. Electrical Systems
At this point, I knew my gauge was not the problem so try to find a way to test the sending unit.
I found this post that pointed me to a way to test the wiring from the gas tank to the gauge. 1997 Jeep Wrangler SE - Fuel Gauge not working - JustAnswer
So I pulled my drivers side rear wheel to get easy access to the connector right behind the rear spring. These are the four wires that (probably) all go to the fuel tank. I disconnected the connector and started to run the test descibed in the attached post (i.e. shorting the two wires to see if it causes the gas gauge to change readings - the blue with the green stripe wire and the brown with the yellow stripe wire).
While cleaning the connector, I noticed a broken wire barely hanging together. After further inspection and striping back the tape and cable sheath, I found that two of the wires were severed and one wire was partial severed. They were severed about 1.5" from the connector (on the engine side of the connector, not the fuel tank side).
I'm not sure how this happened but probably somehow they got crushed by some suspension component or a rock. The copper of the wire was pretty corroded so they had been this way a long time. Luckily the fuel pump wire didn't get cut and fail. The partially cut cable could actually be powering the fuel pump as far as I know.
Well I cleaned up the wires, soldered them back together using some heat shrink tubing a plenty of electrical tape. I then ran the test shorting out the two wires on the connector and my fuel gauge definitely moved when turning on the engine.
I plugged back in the connector and yahoo! - my fuel gauge works and so does the low fuel indicator light.