Rear? alignment on 2005 Outback

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My wife bought a 2005 Outback Oct/04 new. Now Feb/07 the car has 8700 miles, I know it has less than 9000 miles after almost 2 1/2 years, we don't use the car much. Anyway we noticed the right rear tire had been worn almost bald along the outside 2" face of tire. This car has not hit anything or been driven on rough roads. Is there something wrong with the Subaru suspension that allows alignment to change. Now I'm told that I'm going to have to buy 4 new tires because the tires have to match size & I'm not supposed to run mis-matched tire sizes, ie: 1 new tire with the 3 that have 8700 miles on them.