1991 Ford F-150 Fuel Pumps won't cut off

I have a 1991 F-150 with a 300 6-cyl engine, 176,000 mi, dual fuel tanks, and never any problems with performance. I recently got on the highway and the engine light started to come on intermittently. Every time the light would come on, the engine would cut out. If I released the accellerator pedal, the engine would pick back up. As long as I was running at a low RPM and NOT under a load of going up hill, it would run ok. If I give too much gas, it will cut out again. I made it about 20 mi going 45mph. The next day I inspected and changed all filters. Now when I turn on the key, the fuel pump will not shut off. This is teh same for front and rear tanks. I try to crank anyway and the engine will not start. How would you diagnose this problem. I know in other similar cases you stated to replace both fuel pumps. How probable is it that two separate fuel pumps will fail at the same time. It seems that there may be a fuel pressure sensor that may be bad? Please advise!