If it does turn out to be a differential, I would press Subaru as to answer the "why" question.
In terms of repair, I think it would be much cheaper to go with a used transmission or even a rebuilt one. I have had a few AWD cars, but never a Subaru (Newbie). But, if I understand some of the Subie systems correctly, a front differential shouldn't go bad from different sized tires front to rear - only the center differential (or viscous coupling, or torque transfer device in the automatics I think) would be affected. I have run into that on center viscous setups.