Brad have you tried e-go online couriers? (Hunter Express) Their blurb says no car parts but in the fine print it says as long as parts are new (or I take it as clean and just not pissin out oil, make sure seller fastidiously cleans diff before packing) and suitably boxed then they will freight it. I have used them many times for freighting boxed, used car parts.
I just crunched some numbers into their online quoting system.
With their system you can chose from door to door, depot to door or for the cheapest quote if seller is happy to drop off, depot to depot. There was a depot listed in Warnambool (Allens transport I think it said) so I punched in that and used the Cairns depot as your pickup point. I guessed the dimensions of a diff centre in a box at 40cm X 40cm a 50cm with a weight of 21kg. The quote was $174.36 (that may change if I guessed the dimensions incorrectly) I dont know if that quote is good bad or ugly, but at least they have a Warnambool depot, and hopefully close to the sellers house If seller cant drop off, try door to depot pricing.
If you use them, just make sure old mate has a working printer as he has to print off the freight docket to attach to the parcel.
Hope this helps,
cheers, John