1995 Ford F-150 Fuel pump feed fuse blows

My fuel pump feed fuse blows after starting my truck. If I have the tank selector swith set to the rear tank and turn the key to the run position, the pump will activate, I can hear the hum of the pump, and the fuel line will have pressure in it. If I then start the truck with the fuel tank selector switch in either front or rear tank position, it will run for about 15 or 20 seconds and the fuel pump fuse will blow. This tells me the problem is in the circuitry before the tanks, as the problem occurs regardless of where the tank selector switch is set. If I never start the truck, I can hear the pump motor hum each time I turn the key to the run position. I have changed the fuel pump in the rear tank, which is the only tank that has ever worked in this truck since I have had it. Is this a wire problem or a PCM problem? Any ideas? Thanks.