You can go without, but it invites noise and uneven wear. Worth doing while it's apart.
They are service parts through Subaru. Not sure if they sell them individually, as a wheel's worth or an axle's worth kits but you don't have to buy pads or anything to get them.
They are service parts through Subaru. Not sure if they sell them individually, as a wheel's worth or an axle's worth kits but you don't have to buy pads or anything to get them.