If you aren't already aware some of the early 085 gearboxes have pressed on final drive crown wheels and therefore not suitable to fit after market differential upgrades.

A quick and easy way to check yours without ploughing through gearbox codes is to remove the oil fill bung, rotate the diff and look out for this in the hole:

That is a rivit head (one of six) that hold the crown wheel to the diff housing. You will need to drill these out and replace with the VW nuts and bolts if you want to fit an aftermarket diff. On a G40 gearbox, you will actually see the nut here instead. The G40 gearboxes (as far as I'm aware, but don't know about newer D** coders onwards) are the only ones to come with the bolted diffs from factory.

And a bit more info on the dismantling and assembly :-D

WORKSHOP MANUAL GUIDE

Scroll back to the top of that link page for more info.