Go to the Pelican web site, they have a completely detailed instruction with photos. You will need about 4.5 liters for a complete ATF Fluid change.
Ooops, I see you are asking about the Diff sorry I don't have that info.

I figure I would add my experience here since I just did this recently. There is plenty of good information on the transmission change, so I will skip to the differential.

Mine is a 99 tiptronic and there is no drain plug. First off getting the fill plug off whas a beast! I actually stripped the hex head and luckily used an ez out and breaker bar, so had to replace that. Tried sucking out the fluid with a variety of pumps, doesnt work great but better than nothing. With whats in the way in the gearbox you cant suck more than half of it out, so can maybe do tt a couple times?? I had to do it again as i had a leaking differential seal that got worse, and had to remove the axle to replace it so was able to get a full drain on it. I have seen that it holds .8 liters, but in the 2 times I have filled it, never got more than 1/2 quart in? I will suggest giving it some time to settle and topping off to get to the correct level.