To do this to an Audi box you need to swap the output shaft and the rear casing, so it is a full strip/rebuild. The parts aren't cheap either.
On a 4wd box the output shaft is hollow so the rear output drive shaft can run through the middle of it from the diff.
A cheap method is to weld up the front/rear diff, and fit a cover plate to the back of the box, but it is longer too than a 2wd box, and may not fit in the car very nicely anyway.