I've stated a few times that I haven't touched the OEM drive shafts before, and I've never hinted that I've had tranny work done before because there simply hasn't.
I agree it's an odd failure, but it has nothing to do with previous assumed work that had never happened. This driveshaft is stamped "Made in Japan", it's OEM Subaru.
On a brighter note, I topped up the diff with some throw-away Mobil gearbox oil and put a few miles on it. With the new axle, everything worked fine. I'll drain and replace the diff and tranny fluid after a few light miles.
I would have drained the diff fluid entirely, but Subaru decided to use an absurdly big star bolt for the diff drain plug, so now I'm awaiting the arrival of yet another custom tool so I can do an otherwise routine maintenance item.