Well, as the 97+ are famous (infamous?) for instrument panel connector problems you could try pulling the IP and cleaning the connector, apply some dielectric grease, and then see if you can "tug" on the harness to put a little slack in the wiring, then reconnect the connector to the IP and see if that changes anything.
If that doesn't help, then the issue is the gauge, the sender, or the wiring in between.
If you can determine which wire in the connector feeds the sender's resistance to the fuel gauge, then you could buy a resistor at radio shack and test it by releasing the sender's wire from the IP connector, then jumper in a Radio Shack resistor of a set value just to see if the gauge responds.
On the 98 the readings are reversed from the earlier years, but on a Renix Empty was 0 ohm, Half was 44 ohm, and Full was 88 ohm. In theory if you jumper in a 44 ohm resistor you should be 1/2 full on the gauge no matter what year.