There is no switching valve, all the switching is done by the two in-tank pumps themselves. There is a valve on each pump that opens when that pump is energized for the fuel to return to that tank. Sometimes when a pump goes bad this valve sticks open. It sounds like your rear pump is bad. So when your front pump is running, fuel is able to go into the rear tank also.
My 96 does this also. There is a recall for this problem for a variety of years but not for my truck. I called Ford and the lady I talked to said if they get enough complaints they may expand the recall. If anybody else is having this problem please call Ford. It sure seems to me they should fix this.
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