Hey Hoss ,
On my old c6 transmission, autometer and others make a plug in pulse generator that might work with the trucks electronic speedometer. Mine was part of the guage kit from autometer. You just need the gear off the old speedo cable, along with the c-clip that holds the gear to the cable, these parts go right into the pulse generator, which bolts right where the speed cable bolted.
These guys also have them and may be able to help
Speedometer 8K Pulse Generator