My vote too is rear. The reason I would do this if I were you is because with front lockers your steering becomes even worse. So if you have one locker lets say and its in the back you don't have to sacrifice the steering as much just to have your axles locked. Another reason I would go with the rear is because with an aussie locker you dont have an on/off switch, so its engaging can be a surprise at moments. This can be dangerous if you're on an icy corner on the highway, going highway speeds, where you want one tire spinning faster than the other to maintain control. At least if the rear locks up on an icy highway you may be able to save yourself with the steering wheel.
There are some out there that are on a budget and get aussie in the rear and after saving for a while they invest into selectable lockers like ARB for the front.