I believe it is a thermistor.
The BD Xmonitor works using it.
I am thinking as cheap as the transmission sender is for a gage, drop the pan, solder in a fitting, or the you can pull a plug back by the OD sction and put in a temp sensor.
I don't have the factor wiring diagram in front of me.
Another source of info would be the service manual. It probably will tell you acceptable values for the sensor.