Welcome back.
WHAT sensor is this in the gearbox where the plug connects to the back? Does anyone know what it is for and what the part number is for the Jag xj350 j8

 

It is a mechatronics connector. It connects inside electronics to outside. I am not totally sure if gear box ECM is inside or outside of transmission.
To disconnect mechatronics connector you need to turn outside ring of connector CCW. And pull it STRAIGHT out.
​​​​​​there are number of videos on servicing ZHP6, I could quote wrong model here, transmission on U-tube. The same transmission used on BMW, Fords, LR, Mercedes and 'cream' if the crop Hyundai....
Regardless, some different access panels, inside is the same.