Vibrating when accelerating is the inner joint of the axle. If the boot is broken (usually is) then that verifies which side.
1. you can reboot it and the vibration will go away, done it a bunch of times. it's only vibrating because it's out of grease and heating up - not because it's failed....regrease it and you're golden - i am speaking from lots of experience.
2. do not use an aftermarket axle, they're terrible. if you don't believe me, google or search any subaru forum to verify. the original Subaru axles are very robust and should be retained. the rebooted vibrating axle (vibration would go away once some fresh grease is in there), would be higher quality than a high percentage of new aftermarket junk. problems out of the box are common.
3. if you need another axle - buy a used Subaru axle with two intact boots and replace the boots with new boots. i get original Subaru axles all the time for $25 - $33 each and install new boots.
Car-Part.com--Used Auto Parts Market
4. i'll take your old axle if you wanna sell it.
5. dealer axles are really expensive. the only non-dealer alternative that is good - is MWE out of Denver, Colorado. about $70-$90 plus shipping and core for axles. annoying but worth it to avoid the aftermarket debacle.
P0420 code is a joke (in all manufacturers this is an issue, not just Subaru - you can google and read about it all day long):
1. ignore it. zero damage possible. 1980's Subaru's have zero issues even though they don't even have a rear O2 sensor to generate this debacle of a code. the sensor causing this code (rear O2) is not even USED by the ECU, so it's not that it "doesn't" cause issue - it can not cause issues with performance or gas mileage. ***(see note below). drive 100,000 miles with that code on, doesn't matter - it's done all the time and is benign. complete waste of $1,000 to pay Subaru to fix it.
2. install the $5 fix and be done with it. search here or google - very common.
3. if you have any exhaust leaks - fix those and that will likely cure it.
4. installing straight pipe is fine, you just may have the code forever, so you must live in a state that doesn't look for the check engine light.
5. the code will come back, they generally start being randomly intermittent like this, very common. might be 1 week, 1 month, 1 year, 3 years but it *WILL* come back.
6. Subaru has a very detailed smoke-involved machine/process for properly diagnosing this issue...i would never do it in a million years, but be sure you don't take their recommendations without that specific test and results done properly. They may just "guess" or hope it pushes you to buy new.
The tolerances required for this paramter are way too tight and aren't practical for real world usage. What rolls your eyes is that cars like yours which have this catalyst inefficiency code would actually pass an emissions test if you did the gas analyzer, sniffer test. but of course they won't do it with the light on....so, people are spending thousands all over the country to fix this code which isn't actually even violating emissions requirements. It's not polluting - so let's pollute the environment with increased factory, recycling, gas, chemical, and land fill usage. way to be green, high five!!
That's why the $5 fix is so acceptable for anyone that understands this issue.
***(The situation where a severe engine problem is causing some other issues which trigger the P0420 code does exist - but they are almost entirely theoretical and not practical or seen in real world usage. and you would likely have made some comment about your engine being a piece of junk for some reasons)