I wrote this up for another board, but it fits here too. Subie axles are known to fail at about 75k.
1) With the car on the ground, loosen all the hub nuts of the axles you are going to change. These are in the center of the lug star pattern. You will need a breaker bar and an extension to do this, mine was a 4' long pipe.
2) get the car up on jackstands. Only the side you are working on needs to be lifted.
3) Find the axle pin. It will be on the smallest diameter portion of the green part of the axle, near the differential/transmission. It goes all the way through the axle, so there are 2 places to find it. You will want to rotate the wheel so you have easy access to it.
4) Remove the pin. This is the tricky part. You need another pin of some sort of the exact same diameter to hammer on. A pick is no good, it will cause the axle pin (which is a rolled piece of sheetmetal) to spread and get more stuck. I used a drill bit. With the right size tool, this is easy, it is slightly easier if you use the side that has the divot around the pin.
5) remove the axle nut and pull the axle. The whole thing can compress. It is probably a bit easier to take the non-adjusting alignment bolt (the bottom one, iirc) out, so you have more wiggle room.
6) slide the new axle in, taking care to line up the holes so you can put the axle pin back. Reassemble.
I can take pictures of stuff if there is enough interest. I don't really want to though.