chances are you have a bad wire between sender and guage. You could pull guage off corner post and find wires that go to guage and sender. They are normally color coded and numbered. You can ohm check them and find out for sure. if they ohm less than 1 ohm there good. If they are good it is either your sender or guage. You can ohmcheck your sender with is usually a potiometer. Ohm check between the two pins on sender when u fuel it up and reading should change when the tank changes levels. Doesnt really matter what the readings are as long as it changes and it isn't a dead short.
Easy way to find maybe a open or short is have someone set in cab and you wiggle wires down the harness till it either comes on or off and u can sometimes find it that way. Chances are that it is your cornerpost is unlikely, by the way You are explaining it.
