The most likely explanation is that the shaft they replaced was the culprit.
And what major shop doesn't have a Kastar or similar re-threading kit for EXACTLY the scenario they claim caused them to replace the shaft??????
"We're so magnanimous, we replaced a new axle due to damaged threads but not because it was causing a vibration that started as soon as we installed said axle."
It's like a Jedi mind trick (did they wave their hand across your face?):
"These are not the CV axles you're looking for...."
Also as a machinist I find it laughable to say anyone competent would replace a new, GOOD axle due to damaged threads. We have the technology to fix that now....