Hi, and welcome to Just Answer!
The code indicates that the controller never sees the state of the park neutral switch change.
You didn't mention any trouble getting it started, so we can assume that it's stuck in the park / neutral position.
The most likely canidate is the park neutral switch. It's the 3 wire switch on the side of the transmission, just above the pan and to the rear of the shift lever.
It simply screws out, and the new one screws in.
To test the switch, the middle pin should be grounded in park or neutral, and open in any other position. (the two outer pins are for the backup lamps).
I hope this helps, any questions don't hesitate to reply back!
The service information doesn't mention anything about the OD off light coming on, but I suspect very strongly that it's because of the code your getting. The fact that the controller doesn't 'know' that the trans is in gear could have this effect.
(it actually knows your in gear because it sees the truck move, but the fact that the switch is in p/n confuses it.. and when it get's confused one of the first things to happen is OD gets disabled).