The basic transmission is the same - but the tail shaft and rear housing are different - and much shorter. You COULD put a 4x4 tail shaft and housing on a 2wd tranny, but note that you have to totally disassemble the tranny to do so, as the tail shaft is the first piece inside the case on a rebuild.
I've got a used A4LD in 4x4 for sale if you are interested. First $300 gets it.
Don't know much about it, except that it came out of a wrecked running vehicle.