Pull the bed, open the top of the tank ring, pull out the fuel pump assembly and replace the float (It's brass and probably has a hole in it so it filled with gas and doesn't float any more). Check your sender again while you have it out for those same ohm readings.
Ford has floats, they're cheap and easy to replace.
Hardest part is pulling the bed for access. You'll need a T55 for the bed bolts, PB Blaster on the threads underneath a couple days prior and during, a breaker bar to turn the bolts (you'll need it), remove the 3 screws/bolts for the filler neck attachment, and don't forget to disconnect the tail light wiring.
2 guys can lift the bed off.