Fuel tank switch.
I have 1989 F-150 4 X 4 with a single tank. It was optional to have one or two tanks. There are aftermarket switches to replace the original although i don't know off hand where you would find one unless you google the heck out of it. Also try obsolete ford motor company and wrecking yards. Also check for newer model trucks with dual tanks. I have found that some/many into the 90's used the same parts and on single and dual axle trucks and vans. Hope this helps