Before going to all the trouble of droping the tank and all go hit up a local parts store and pick up a couple of cans/bottles of either Chevron Techron FUEL SYSTEM CLEANER [not Fuel injector cleaner] or SeaFoam and dump it in your tank. Often that will clean the valve up and allow it to start functioning again. The ones I have looked at were generally crudded up with a varnish looking stuff. I use either chevron, valvoline or seafoam fuel SYSTEM cleaners every 5-8000mi.
Dropping the tank is a PIA, figure 4 hours in the driveway, half hour or so more if you have skid/hitch to free up the mounting bolts on those. If you do have the hitch or skid soak those bolts a week in advance, 2x a day and you might even want to go get a new set of the plates for inside the frame rails, when I put my tomken rear bumper on two years ago those nut plates were in not so good shape. If it had been a weekday I would have ordered a new set and used them while I had the bumper off. Those bolts had been in and out yearly and had been anti-seized more than once but the plates and nuts themselves were all corroded. Next time I get the plates they are going to get sprayed with a better anti-corrosive paint.
Actually thats a good idea, think I'm going to order a set today and prep them, just in case my fuel pump fails, can't hurt to have them on hand.