Engine stalls, p0720 engine light on. Runs ok than goes sporadic.

There are 2 possibilities here:
1.- There may be a problem with the new speed sensor, the electrical wiring between the speed sensor and the PCM, or an internal PCM problem. To see if VSS signal is present at the PCM a scan tool capable of displaying live data is required.

2.- There is a recall action on this model year for erroneous speed sensor signals; the cure is an updated software calibration. If your truck has not yet had the updated calibration installed, you should call your local Dodge dealership with your VIN number and ask the service department to check to see if there are any open recall actions on your vehicle. If the software calibration has not been updated yet, they will perform this procedure free of charge for you. If you do not have the current software calibration in your PCM to correct the VSS issue, you need to have this done before proceeding any further with attempting to diagnose this problem

This sensor is located at the rear of the transmission.

Symptom: P-0720 LOW OUTPUT SPEED SENSOR RPM, ABOVE 15 MPH
When Monitored and Set Condition: P-0720 LOW OUTPUT SPEED SENSOR RPM, ABOVE 15 MPH

When Monitored: Continuously with the key on, engine running, and gear selector not in park or neutral.
Set Condition: This code is set if the vehicle speed (as determined from the vehicle speed sensor), is above 15 MPH, and the output shaft speed is below 60 RPM for 2.6 seconds.

POSSIBLE CAUSES DTC CODE RETURNS:
OUTPUT SPEED SENSOR DEFECTIVE
PARK/NEUTRAL SWITCH CODE PRESENT
OUTPUT SPEED SENSOR GROUND CIRCUIT OPEN
OUTPUT SPEED SENSOR GROUND CIRCUIT SHORT TO GROUND
OUTPUT SPEED SENSOR SIGNAL CIRCUIT OPEN
OUTPUT SPEED SENSOR SIGNAL CIRCUIT SHORT TO GROUND
PCM DEFECTIVE (DRB SHOWS > 60 RPM)
OUTPUT SPEED SENSOR HARNESS OBSERVABLE DEFECT
OUTPUT SPEED SENSOR WIRING HARNESS INTERMITTENT DEFECTIVE

Hope helps.