
So after spending a couple days tracing wires, tracing more wires, checking for power in wires, and checking a few random relays, I noticed a small black wire near the battery that didn't go anywhere. In fact, it appeared to have corroded to breakage, and there seemed to be a connector (also used by another wire) that it might have come from. But I discovered this last night around dark, so I postponed the investigation.
This morning, I stripped the plastic off the connector, and found one attached wire and metal strands from another wire. Figuring that was at least a problem if not the problem, I biked out to RadioShack, got a new connector and a digital multimeter (which I didn't use, but I've wanted one for a while), reattached the two wires, and lo and behold, the truck started! :)
More technical info, in case you're idly curious:
Upon talking to my dad, who'd spoken with someone better versed in Bronco repair (who says the Chiltons manuals are much better than the Haynes), I learned more about the power supply to that fuel pump. Apparently there are two power sources--one from the starter and another that runs through the relay and crash sensor. Thus, the pump runs from the starter-based source for a few seconds as you're starting the car (On and Start), then shifts to the "Is the car running, and has it not crashed?" source. As it is, the power for both of those come from the broken wire that I repaired. :)