Most tranny shops would chargbe BIG money to disassemble the tranny to replace that seal. Most would not even replace just this seal, they'd want to rebuild the whole thing for like $3000. Not an option for most people.
Subaru FSMs show the disassembly as a very tough process, involing removing all the guts of the center diff, etc....ughh.....
Thankfully That is not needed to replace the front seal!!!
*****It's worth noting that I also replaced the front shaft bearing (the one with the dowel) since there had been play on that shaft. The pilot bearing in the flywheel that supports the very front of the shaft had grenaded, so the bearing in the trans was doing all the work. I'm sure it was wobbling causing the seal to go. I believe it was also making for crunchy shifts. It was $34.17, and replaceing it was super easy while apart and made for better shifts.
It also would be a good time if one was inclined to replace the diff/axle stub seals, as this way you don't need to remove their retainers or reset backlash. As well the rear output seal and the crush washers for the neutral and reverse switches are accessible and would make for a FULL reseal. *****
However this writeup covers only the front seal replacment as it is the only one that HAS TO BE done this way.
Here's the skinny:
First remove the trans:
Next, remove the four bolts on the coverplate on the rear. You'll see the shifter shaft, and teh little arm that locks it into the reverse checker(lockout) remove the 10mm bolt that holds that arm in place, careful not to drop it down in there.
Now remove all 8 14mm bolts holding the rear section to the front case halves. Rotate the shifter rod CounterClockwise to disengage it from the forks and then slide off the rear section.
This is what you'll see. Take note of how the 3 shifter rods notches are alinged, as you'll need them this way for reassembly. All the slots line up. Here's what the *finger* on theend of the shifter shaft that engages them looks like. 
Remove the 2 bolts on the right(as installed on car passenger) side in the bearing retainer around the lower shaft
Now it's time to open the case halves. Set it on the left side, you'll be lifting up the right half (left half has all the shift rods and forks in it) Remove all the bolts around the edge, and 4 14mm ones that are in the middle, 17 in all. Make sure you haven't forgotten the two that I mentioned above in the rear where the shaft retainer bolts on. Double check that you got all the bolts, and split the case.
Here is the seal you are after.
Lift out the shaft
Slide off the seal, and slide the new one on.
Now, there are a few important notes to the reassembly. First, there is a dowel pin in the recess for the bearing in the middle/front of the shaft. The hole in the bearing must seat onto this pin or the case won't go back toghether right.
And, at the rear of the shaft, there is a snapring around the large bearing. it only goes about 2/3rd of the way around, leaving an *open* section. That open section must be towards the lower shaft. Otherwise, the ring hits the lower shaft retainer ands again won't seat.
Slide this collar back on. It just sits there, and it's pretty obvious which side the bevel goes on.
Now bolt the 2 halves back toghether, make sure to use sealant (I used threebond, Grey or black RTV would probably work.) No pics of that, but once it's toghether stand it on end. You'll need to set it on something with a holein it so the input shaft can stick through. I used an old wheel.
Now, there is a real tricky part to get the shifter shaft to line up with the selector rods. First, pull all the rods so they are in *neuteral*, the middle notch of their travel. It's pretty obvious when all the slot are aligned.
Apply sealant (or a new gasket) to the edge. And get ready to drop the rear section on. Now as you drop the section on, you need to rotate the shifter shaft CC. Then once it's seated, you can rotate it back to where it engages the rods. It's tricky, and I wish I coiuld take pictures but I was by myself. Took me 4 tries to get it right. I kept getting the shaft to far in, and catching behind the openings in the ends of the rods and moving one before the other engaged.
Once you think youi've got it, put the interlock bolt back in,
then try shifting to all the gears. If you can't get all six positions, remove and try again. It sounds hard, but once you've got it all in front of you it becomes pretty clear.Total cost of seal replacement = $12.77 for the seal and a fraction of a tube of Three-bond.
