If it is the same as the 89 there are three fuel pumps - one in each tank and a high pressure pump which feed the fuel rails. This is not the first time I have alerted US F150 owners to the structure of the fuel system (I am in Australia where Ford stopped making F150's in
1989 - I have one of the last). My F150 has a low pressure fuel pump in each tank. These feed to a gadget located behind the high-pressure fuel pump - the one which feeds the engine fuel rails. The gadget - called the dual function resevoir - is purely mechanical - whichever tank is switched on directs fuel which causes a diphragham in the DFR to close off the other tank and allow the high pressure pump to draw from the chosen tank. It is a neat system which, occasionally, causes problems which are difficult to diagnose.