You don't have to drop the box oil just to change the sensor... You just need fast hands and you'll lose about 50ml of box oil. If you jack it up enough on the passenger side you won't lose any.
Health and safety notice: Do not attempt this work shortly after driving the car for hours. Gearbox oil gets very hot and will make you use naughty words if it touches your skin so allow it to cool.
Mick
+1... I did mine under the car a few weeks back early in the morning when we had a frost & the oil was nice & thick.
Jacked the N/s up, slid under the car on some cardboard - span the old sensor out, stuck my finger over the hole and span the new one in.
Sbridgey - if you have a new n/s driveshaft to go in then I would wait til then, access would be MUCH easier... had my driveshaft out last week to sort out the infamous JB3 diff circlip curse.
Sensor just to the left of the driveshaft hole, white/black striped lead.