Sounds like a ground issue or the connector on the back of the cluster has shifted and making contact where it shouldn't. I'd try pulling the cluster out first and pull the connector and re-attach it and see, also look to see if the ribbon cable on the back of the cluster is heavily scrached or cracked.
Casue those 2 systems have nothing in common electricaly other than at the cluster, as the voltage is sent to the 4wd indicator the fuel level drops if I understand that correctly. Try removing the bulb for the 4wd and see if it does it after you reconnected it. If it does with the bulb out of the cluster I'm outta ideas, if it doesn't theres a problem on that ribbion cericut on the back of the cluster.
Edit: I just had to check to make sure,
The Jeep fuel system to register level, goes from low voltage at FULL to High voltage at Empty, I know that doesn't nessacarilly make sence but how it is. So what happening is the 12v is being applied somehow from the 4wd indicator being lit, and is crosstalking to the circuit the fuel guage is on. Hence sending voltage, and the guage thinking that its reciving voltage from the sending unit, when is comming from another circuit.
My two best guess now, 1) problem in the circuit on the back of the cluster 2) The connectors on the engine computer were removed and pushed back on and a pin or two got bent.