What would make a truck not want to shift into OD or lock up?

 

It might be one of the sensors in the rear end, trans, or the crank position sensor. When my crank position sensor went out I also had lights on the dash coming on and no gas gauge along with no OD

 

there are a lot of things could be wrong to not make it shift. fluid and filter should be good, make sure the throttle pressure cable is adjusted, then a scan of the codes should point you in a direction if there is an electrical problem. if not you just have to start testing . the tps, output speed sensor, speedometer, tach and governor solenoid have to give the right signal to switch the tcc and od solenoids.
if you need specs pm me and i can help with diagnostics.

 

Check with Dave. Surely can help on his tranny.