Interesting.
The speed sensor on you your truck is most likely the magnetic pick-up on the top of your diff. Is this the part you re-sealed? That pick-up sits very close to the ring gear teeth. Its possible that it is now sitting too far away from the ring gear for a proper signal. I suppose it's also possible that re-sealing the diff case to the axle now has too thick of a seal causing the same issue, but unlikely. As far as that pick-up, it's not RTV dependant.

85 4Runner - With NEW Marlin 4.7 Gears!! It's a whole new beast!