If the TCM was already refurbished and the P0791 code is still present, there's a chance the input and output speed sensors are faulty. That, along with dirty transmission fluid and filter can throw the code P0791.
If the transmission fluid and filter check out ok, meaning the fluid is bright red and filter looks clean, and the two speed sensors were inspected and/or replaced then there's a possibility that the fault is with the TCM still.
A hard battery reset is strongly recommended, below are instructions and a video on how to perform the reset.
Hard Battery Reset:
- Remove the key from the ignition.
- With the battery removed, to ensure there is no power, short the battery cables in the vehicle and connect the positive and negative cables together for at least 20-30 minutes.
- Reconnect the battery cables starting with the positive cable on the battery first.
This will reboot all the computer modules including the TCM and make the vehicle relearn its own values.
It's recommended to have the vehicle idle for a few minutes then take it for a drive at least 15-20 minutes after the hard battery reset.
It might idle rough at first and shift strangely but it should smooth out after the drive.
If you're not confident in doing the hard battery reset, simply leave the car battery disconnected overnight. The main goal is to drain the voltage in the computer modules to reboot them.
Video: