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Rear left
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Rear right, it blew out...
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Front left
I can say with absolute certainty, as I dropped to my hands and knees for inspection, that 2400 mi. before the blowout, the rears had more tread than the front ones. This lulled me into cursory inspections during the trip and everything appeared ok. Actually I was more worried about the Goodyears in the front rather than the Michelins in the back. I was planning to order Michelin PS4S N0 for the front axle (well, I did... after the blowout).
My gut feel is that the wear happened suddenly, possibly in the last couple of days, as a result of out of spec toe in (approx 0.3 of a degree symmetrically on both tires). Not having inspected the tires closely in the last couple of days, I can't be categorical about it. Just grateful that I got through it with no major consequences.