If you have an S80 gearbox, easiest way to find out if you have a LSD box is jack the front of the car up and spin the wheels. If they both spin in the same direction, you have a type R box. If not, you have a VTi-R box.

I would personally do a LSD before a final drive as it will make the car alot easier and safer to drive.

Where are you located so I can refer you to someone that does gearboxes.

If you are in Melbourne, drop by and say hello. Otherwise I will point you in the right direction