i think you would have to use an F series housing and axles and have em shortened to the au width, the rear track on an au ute is pretty wide, would be wider than a xa-xd, so one of those diffs wouldnt suit....
im pretty sure the hduty au diff axles are 28 spline like the 9'..
they would just use these , and make the housing to suit.
It's certainly been done (I can't help you with any useful info re what, where, how though) and at least it's a ute - imagine how much more complicated it would be if you had the Watt's Linkage suspension! So long as the axle housing was reasonably similar in width to the normal diff (as philstoj said it's likely to be a bit narrower) you may be ok with just using different offset wheels, eg EL wheels rather than AU.
Since TIKFD6 doesn't mention a 6 speed in his sig, I'd assume he decided he didn't need one that badly once he found out the price - likewise do you really need a 9" in the first place with the 6 cyl? They are also a lot heavier than the normal diff as well.
Very True Outback, i also considered a Hydrotrack centre for my diff, but these are expensive aswell.
But they are very good.
As you would know if you drive an XG/H Outback ute.
If you are interested PM Tickford2001, he has put a Hydrotrack centre in his ED XR8 Sprint.