Dojj, its not actually the driveshaft that sends the power into the transfer box, it comes off the diff directly, it just looks like its the driveshaft as they connect together.

Well thats based on:
a) a really really good understanding of the vauxhall cavalier setup
b) the fact that when I looked at a 4wd transmission a mate of mine had it looked the same

So im only 99% sure im correct not 100% so someone correct me if im not, but if you think about it, it would be ridiculous to hang the entire rear end off the output from one side of the diff like you are saying, as if the front wheel on the other side span no power would go anywhere else.
Where as what I am saying is its driven off the same output from the box that the front diff is driven, so the box is essentially connected to the front diff and the rear VC seperately.