It could be the gauge or it could be the sender in the tank...
The Sender is grounded just behind the passenger's side kicker panel up front (a WHITE-BLACK wire), and the signal wire for the fuel sender is a YELLOW-RED wire. The Inst. Cluster has it's own ground, and if it was bad then nothing would work, so don't worry about the ground there.
Also, you might want to check the line that runs from the tank and up into the cab. Did a rock hit it? Did it melt when you were welding on some rock sliders or something?
Good luck!
BigMike
