the part no on japanparts is 1005G-028-071 and its 17,800 yen. So it'll probably be around the AU$300 mark landed.

Labour is easy. Drop Oil. Diff out. back plate and side bearing carriers out. LSD out. Rattle the crownwheel bolts off. Crownwheel off. 4 Bolts off to separate the the 2 halves of the LSD center. Replace all the plates inside with the kit. Lubricate the plates as you layer them in the LSD casing with gear oil. Assembly is reverse of what I just said. Use locktite on the crownwheel bolts but not the 4 bolts that hold the 2 halves of the LSD together. Those 4 bolts dont need to be very tight as they just hold the LSD together until the crownwheel goes on and really clamps it all up.
There's no need to muck around with adjusting the backlash if you are not changing the bearings. Just make sure the shims that pull out with the side bearing carriers stay with the side they came off with.
You may want to use this chance while the diff is being re-assembled to put in fresh shaft seals and O-rings in the side plates. Clean out as much of the old oil while the diff is apart and then put fresh stuff in when you re-assemble it. Use the OEM metal shim gasket for the back cover plate and dont use any gasket goo or silicone to seal it. Just torque up the cover plate bolts properly 1 quart of oil needed for re-fill.

The LSD will need breaking in once fresh plates go in I think. I don't know what Subaru recommend, but I remember reading Cusco recommend doing slow low throttle figure 8s for their LSDs so Id imagine it would be ok to do the same with a STI plate LSD. Check with Doug. Im not sure if you need to drop the oil and then put fresh oil in that you will actually use.

__________________
RS Liberty - Because WRX's are only good for parts...