As far as I'm aware, you can get away with changing one axel at a time... it wont wind the diff up, but have always thrown on a set of 4 tyres each time one of my Imprezas has needed replacements.

Suppose a 'normal' day to day Forrester isn't going to get the kind of treatment that an Impreza Turbo or STi is going to get so not quite as important to treat mechanical parts with as much mechanical sympathy.

Saying all that tho, I know people that have rolled around on different depth/age tyres on the same axel with Imprezas and they haven't had problems.


Cheers,
Grant