Have you considered that the sending unit may have failed?
Just look at a couple of listings, Geno's list at $95 Ouch! Others are higher,
You should check the wiring. Looking at the Haynes manual, it is a Grey wire with a yellow trace that goes through two connectors to the gauge. The other lead Grey only goes to the warning light.
01 2500 5.9 QCLB 4X4 5 Spd, Valair DD Organic, Power Puck, 268K miles ISSPRO Gauges FP, Pyro,& Boost. "A" Pillar, FASS DDRP-02, BIG Line Kit, 18 PSI, 3.55's, Kool Vue Tow mirrors, Bilstien 5100's, Pacbrake, Puke bottle deleted.