I swear this question pops up like every month. It is a common problem and NO ONE on any psd forum has ever answered it. Mine does the same exact thing.
If I ever get around to it Im gonna do a full right up with pics and a video on how to fix it. The fuel sending unit is the culprit. You can either drop the tank or pull the bed off. I havent decided which one to do. Either way you can kill three birds with one stone when you are inside that tank so it is worth it (sending unit replacement, hutch mod, and harpoon mod) search the forums for what those are.
Some say beat on the tank with a hammer, drive over rail road tracks, SCREW that ive seen a couple of trucks with stuck floats and that fixed the problem but 80% of the problem is the bad sending unit. Id check the wires real good, ive heard a bad ground could cause a false reading also. Just some thoughts, when it gets wamer and life slows down Ill try to fix it with a write up.