i just bought a 98 12v of a guy in TX, and it does it too, heck, he said it has been doing it since it was new. from what ive read, you have to lift the bed or drop the tank to switch out that sending unit. i just ignore it, it lies on the low side, not the high side.
Most of the time a guage not working is simply the sending unit. I know a lot of Chrysler vehicles are known for this. In my opinion it is easier just to take the bed off as long as you have a couple of buddies to help lift it. Its 8 bolts, tail light wiring harness, a ground strap to the body, and your filler neck.