Just had this happen on my 02. Couldn't get gas into it. The pump would shutoff after about a gallon went in.
Shop checked it out and said it was the evaporator canister. It is big plastic tube like thing about 6 inches in diameter and 18 inches long. It is located up under the spare tire. They said it was plugged up and wouldn't let the air out of the tank. I don't understand how the system actually works (or how it could get plugged, Ford calls it a no maintenance, lifetime part); but, I think the tank vent goes to the canister and then from the canister to the vent in the filler pipe so that once the fuel cap is in place it is a closed system. Must be some additional plumbing in there for the purge valve.
On mine, I think it got filled with fuel somehow when the tank was dropped to replace the fuel pump and air wouldn't pass through it.
I think you might be able to pull it out and test it to see if you can blow or suck air though it. I used a shop vac. to, more or less, test it. I'd also check the lines to/from it for kinks that would keep air from flowing.
Anyway, changed it and problem went away.