I beleive the problem lies with your fuel selection system. The fuel pumps keep pressure on the fuel system, any fuel not sucked into the fuel rail and burnt is returned to the tank. Each tank has it’s own send and return. Now what’s supposed to happen when you switch tanks the solenoid/selector valve is supposed to switch both the send and return line. No issues when you return fuel to the tank your sucking from. If the send switches but the return doesn’t, then you have the issue your having. I believe your truck has one valve for both lines and the fuel selector switch controls them both. So it could be the switch or the valve. Also if anyone has worked on it, they can be switched at the valve. So you could have your returns reversed. Fill up the front and run on the rear see if the front overflows. If itndoes just switch the lines. Otherwise the valves is 50-75 and the switch is cheap too usually. Good luck!