What you describe are "classic" symptoms of a failed crankshaft position sensor. Oftentimes, those two gauges will not work when the crank sensor fails in a certain way.
Here is how to further troubleshoot this. Disconnect the connector for the crank sensor and if your fuel and voltage gauges start working with the crank sensor disconnected, there is smoking gun proof that your crank sensor is shorted.
The connector is on the right side of the engine as you are facing the engine and should be near or slightly below the firewall. If you have to buy a crank sensor, do yourself a big favor and buy a Mopar unit directly from Jeep. Crank sensors are not all created equal.
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Past Jeeps: '49 ******, '81 Scrambler, '88 Comanche
Current Jeep: 1999 XJ Sport, bone stock, daily driver.
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