Speedometer not working

[Spedy7]

This has been bugging the hell out of me ever since I bought my truck. As far as I can tell, the drive gear on the transmission is fine and secure, the driven gear isn't stripped, quite certain the gears are mating properly, the speedometer cable feeds to the driven gear fine, the speedometer cable isn't broken, and I'm pretty sure the cable is connecting to the speedometer. Turning the driven gear, it turns the speedometer. Jacking up the rear end and spinning the driveshaft turns it, yet unscrewing it from the back of the speedometer, the cable doesn't turn.

Me and my dad are puzzled to what would be wrong. Already bought a new cable thinking the inside cable was too short and not feeding the speedometer or driven gear properly. It's a 6 cyl truck with a 350 and TH350 swap from what I'm guessing is a later year, but shouldn't be problem. Any ideas?