I am posting for my father (I am a newb with very little knowledge of vehicle related stuff but learn well and my father is intermediate i would say with his knowledge) who owns a 1999 dodge ram 1500 automatic transmission 5.9L Laramie 4x4.

A P0720 error code showed up
The truck won't shift out of low. A few hours before it happened everything was working fine except for when shifting the place on the dashboard that shows which gear you are in wasn't displaying properly, for example when shifting into drive it would say its in Neutral even though it was in drive.
The previous owner replaced the transmission already and that's all the details that I know about that.
I'm happy to answer any question or provide any missing information.

 

P0720 Output speed sensor circuit. PCM isn't getting a signal from the output speed sensor. So, it doesn't have a clue when to shift, therefore, it doesn't. Might be a bad sensor, might be bad wiring, might just be a bad connection. Output speed sensor is on the drivers side of the trans kinda toward the parting line for the O/D housing. I think.... Verify it is plugged in good, and the connections are clean. Go from there.