That's your DRAC, and it'll be located behind the glovebox in the same area as the ECM. It's a small circuit board that is cased in plastic. You can get them from the junkyard all day long out of other trucks for next to nothing; maybe $25 max. The trick is picking one that comes from a similar truck as yours with hopefully the same transmission and rear end (check the RPO codes in the glovebox) - the DRAC is "adjustable" for the differences and with the wrong settings your speedo will read incorrectly. There's a tech article here on FSC that describes modifying the DRAC with a dip switch to make it easier to adjust, as the factory used jumpers that have to be soldered/cut/etc. to make changes.
Also don't rule out the sensor in the transmission tailshaft.