Start with the speed sensor, its the only sensor on the OD section of the transmission, drivers side. Take a 1" socket to get it out.

Sounds like you could have a stuck TV motor. We have been seeing more of these here recently. You need to clear the code and then pull the TV motor off, but still plugged in, and see if the code comes back when you start the truck, unless you have access to a scanner.

 

Pulled it out and it worked fine put it all together and now about 6 months later it did it again. I think i need to replace it.Stuck TV motor that is.