The gauge on my 91 (2.3 standard) acts like the tank is overfull and moves around a lot on the upper end, even when tank is half full. Not sure how to diagnose the problem. I know how to access the fuel pump but not sure if that is necessary. Any tips welcome. Thx.

 

Best way would be to pull out the cluster and test the Yellow/white wire from the gas tank's sender
Have the tank over 1/2 full, but not full, for the testing
1991(1989-2011) senders use these OHMs
16 ohms = Empty
158 ohms = Full

With ohm meter hooked to yellow/white wire and grounded in the cab you should see ohms between those numbers
Now rock the truck so gas in the tank sloshed around, i.e. float goes up and down, and ohms will change as it does, you WILL notice if its too "jumpy", if so then sender has dead or shorted spots
If it seems smooth then replace the "anti-slosh module" on the back of the cluster, or by-pass it, Google: Ford anti-slosh module
Loads of info on doing either