See this info fragmented all over the place and thought it would be good to summarize in one place.
Rule #1: Do not use an aftermarket axle unless it is from MWE or rebuilt from RAXLES.
Rule #2: See rule #1.
A1 Cardone axles or "other" from Autozone, Advance, O'Reilly etc simply allow too much lateral play. They suck. You will have problems.
Rule #3: Given that rules 1 and 2 are Gospel, if you're rich, buy OE Subaru axles from 1stsubaruparts.com. If you're not rich, buy junkyard axles and re-boot them if the boots look dry/old. If you own a 2000-2004 where the passenger inner boot sits above the catalytic converter, you WILL be replacing that boot every few years. It's not hard, just get used to it. A buddy, 12 pack of beer, floor jack, breaker bar and a few sockets are all you need. Small price to pay for an AWD car that will run forever if you take care of it.
I can personally testify to the fact that ALL vibration problems went away after chucking the aftermarket axles in the trash and putting on OE axles that came from a junker with 100K miles on it. There is no substitute for the OE axles. Period. End of discussion.
Rule #4: Obey rules 1, 2 and 3.