The sending unit and gauge are set up for ~70 ohms resistance from the sending unit is empty, and ~10 ohms is full. The stock sending unit has a two-wire plug connecting it. One wiere comes from the gauge and the other is a short ground wire. Remove the plug and the gauge should drop to empty. Short the wire from the gauge to a ground (paper clip between the two connectors in the plug) and the gauge should peg over full.
If this happens, the problem is the sending unit. If not, the problem is somewhere in the rest of the Bronco!-
:-X (wiring, gauge, ect).
If you're handy with an ohm-meter, measure the resistance of the sending unit with the tank full, and with it close to empty and see if the resistance changes.