I think you'd be saving yourself some work if you just dig into your existing setup. Set it up on jackstands, grab the tire, and try lifting it up and down, and pulling it in and out. If there is any play, it could be just as easy as a wheel bearing. Then drop the shaft, and check the pinion like was discussed above. If that doesn't point you in the right direction, drain the gear oil, and look for metal shavings. it's not that hard to drop out the 3rd, so that would be the next step. Could also be the Aussie has given up the ghost and is just whining at you. They don't die often, but they certainly can.
I think your asking for a whole bunch of fab work to replace the axle housing, which may create more problems for you later. Of course, this is a good excuse to upgrade.
Erik
