PROBABLY, each tank has it's own low-pressure fuel pump, only one of which runs at a time, depending upon which tank is selected. These low pressure pumps feed the high pressure pump in the frame rail. If this is the system you have, there will be a weird cannister thing, also in the frame rail. This unit contains a diaphragm, which is acted upon by whichever tank pump is running. When the diaphragm is pushed one way or the other, it moves a valve, which switches the return fuel to the same tank it came from. It can be a troublesome system. One or more of the pumps may have failed, as well as the cannister thing with the valve in it. Also, I have seen the wiring melt off of the pumps, or where it passes though the sender/fuel pump unit, to the outside. The relay should be on top of the driver's side front fenderwell, or in that general area.
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