Do an OHM test on the sending unit.
1) Fold your back seat forward and move carpet back. You should see a circular access panel in the floor on the drvrs side. Open this (4 screws) to access your sending unit. The sending unit with a full tank of gas should give you a reading around 110 OHM's-1/2 tank 55 Ohms- empty 1 OHM...
You can even remove the sender to check its operation. Through the access panel. Clean any sand, rust,or leaves off the top of the sender....(don't want that trash in your tank) Disconnect and put some screws in the fuel line and vent line to prevent anything getting into them disconnect the ground wire and guage wire.....Using a flat screw driver and hammer tap the outisde of the retaining ring counter clockwise. Remove the retaining ring. You should now habve a straight screwdriver and needle nose pliers close by. You may have to disconnect the float from the sender to get it out....gently use the screwdriver to pop the float out of the retaining clips being very careful not to drop it into the tank.
2) Check the senders ground. It is grounded with a 2 other wires on the drvrs rear frame.....The sender also uses two pigtails to connect itself to the wiring harness ...I'd clean those connections.
3) This is the one that's a little more tricky. The wire going to the guage I believe is TAN it could be shorting out... inside the wiring harness running along the frame. That being said....the wire loom runs between the frame and the gas tank. You may have to drop the tank...it isn't that difficult unless you have a full tank of gas....if so drive it around the block 2x it should be empty then
I'm still chasing my fuel sender goblin and will update this for more information as I go.
let me know if this helps. jim
Keep on Waggin
87 Grand Wagoneer w/Sunroof ...BJ's 2.5" Lift,Tadrack, Dana 44's, 3.31 gear ratio, Warn SS diff skid plate, 31x10.5 A/T BF Goodrich tires, SS brake lines. Rebuilt 360, Edelbrock 3731 intake, 1400 carb and Z&M HEI Dist. 260H Crane Cam. Frame sandblasted and painted w/truck bed liner.