Tires: Outback Tire difference, Subaru Tires Matching

Question
I ruined a tire and had to replace it in my rural community.  Our local dealer carries Firestone tires, and could not replace with a Michelin to match the other three tires.  Three months later when I went to rotate the tires at the place (four hours away) where I initially bought the Michelin set, they impressed upon me the importance of having four of the same tires on my AWD Outback.  Soooo I went home to the local dealer and said "Maybe I need three new tires to match the one I bought recently.  They did some measurements, and found that there was a 3/32" difference in tread depth between the new one and the two best tires.  There was more tread difference on the third tire, so they recommended that I change the one tire, but not the other two.  So now I have two old Michelins on the back and two Firestone tires on the front.  When I went to pick up the car, they said it test drove just fine, but that I do have a 1/2" difference in circumference between the different tires.  They think if I am going to have transmission issues, that they would already have surfaced, but that on the other hand, the difference in tire circumference may be significant enough to warrant buying two more to get a matched set.  This is a 2000 Outback, and the Michelin tires (Symmetry?) have not been rotated on schedule.  They have about 30,000-35,000 miles on them, and were probably 60,000 mile tires when I bought them.  The Firestones (Fuzion HRI)were the less expensive option at the time.

Answer
Vivia,

I have heard horror stories about Subarus with odd tires on them.  You have pushed your luck way too far and it is time to get 4 matching tires - IMMEDIATELY.  It is possible you have already done damage to the center differential, but let's hope not.

BTW, the cost of 2 tires may seem high, but compared to the cost of thousands of dollars on a new center differential, it is cheap insurance.