What I said over here might apply to your situation:
... a mismatched tire won't break your drivetrain immediately. It's more
subtle than that: at least one differential will be loaded all of the
time and at all operating speeds. So, instead of managing the
different rotation speeds in a relatively low speed turn, you'll be
cooking your differential at highway straight line speeds.
Now, your situation might not be the same. Here's how to tell:
How bad is the tear? Did it chip one of the tread blocks? Or is it effectively through the tire and causing a leak? If more that a chip and less than a tear, how worried are you?
How worn is the tire? How much tread has been used up? Can you even tell that the tires have been used? Or can you see all of Lincoln's face?
In the end, if you are talking about super worn tires, you should replace the set. If they're barely touched, you might be able to get away with just replacing the one. Somewhere in between, you might be able to replace the front pair. You're going to have to make the call.
That said, you should listen for the audible symptoms of a stressed differential:
you can hear the differential being overworked if you listen for it.
On my Subaru, if I inflate the rear tires to match the front tires'
pressure (approximately 4 psi up), I'll hear a distinctive
high-pitched whine from the center differential every time I coast
down from speed in second gear.
As I said before, my WRX and your Outback are not the same car. However, the vehicle will try to let you know if there's a real problem prior to total system failure....
