Probably the sending unit for the guage. It's located in the tank and it's expensive since it has the primer pump as part of the assembly. Same part as the 740 though so it may be available aftermarket.
Check the tank area to see if any wires may have been damaged recently, maybe from a heavy/awkward load. Make sure the connectors are all seated properly.
If you've got a wiring diagram, find the sending unit connector in the trunk. You could try shorting the line that runs to the guage to ground. This should force the guage to read full and confirm that it's all working in that direction. (use a buddy to monitor the guage - don't leave it shorted too long, you don't want to damage things)