I'm having a hard time understanding why this is a CV joint problem.

For the CV joints to have failed and caused this metal-on-metal noise, which sounds, from the description, like one piece of metal rotating continually against another piece of metal . . . the CV joint(s) would have had to suffer a catastrophic failure.

Normally, CV joints give you weeks of warning, even months of warning. To the best of my knowledge, they rarely fail catastophically. For the CV joints to be fine one day, and then fail the next, just doesn't make sense.

I would suspect a torque converter failure, or a flex plate failure, or something internal to the tranny.