Wow. Your fuel light works? My 1986 Trooper tells me when i first turn it on how much gas and then will show zero sometimes while driving. You have a sending unit/ float in the gas take that sends signal to the gauge. Could be as simple as a bad ground on the sending unit or gauge. Old gas could be coating the contacts on the sender or bad sender or gauge. Multi-Meters and Circuit testers are your friends. Good luck fellow troopererer.

 

The sending unit typically goes bad before the gauge would. IF you can get it out of the tank you might be able to clean the windings. But the bolts may be too rusted to get it out.