Using your open ended wrench to hold the bolt head (it's behind the axle) and your 17mm socket remove the nut from the bolts at each end.
You will not be able to remove the bolt because it will be blocked by the axle, but we have a way to work around that.
Take the nut from each bolt and thread it onto the stud.
Tighten these nuts down so that you have now double nutted the studs. This locks the original nut in place so you can back the double ended stud out of the rear diff. If you don't do this, and just try and back out the stud using the single nut you MIGHT get lucky and back out the stud, but make life easier on yourself and follow our tip to double nut it.
Now, here's the tricky part. Using your open ended wrench put it on the original nut by sliding the end of the wrench between the rear diff mount and the rear subframe.
Being careful to not round off the nut, back out the stud. It'll take some effort the first time, and remember "Lefty loosey, rightey tighty."
On the STi be VERY careful about the rear diff sensor.
Once you have the stud out you will notice that the threads for each end are different. Course threads go into the rear diff and fine threads are for the nuts.