If it leaks fuel at or near full only then it is probably the large plastic nut the holds the fuel sender and hose assembly in the tank. It may just be loose. You can access it with a long screwdriver by looking over the frame rail under the fender. Give the nut a few licks with the long screwdriver and a hammer on one of the lugs. See if that helps.
If that slows it up but does not stop it, then you need a new gasket.