We got to the back and I discovered the REAL culprit, though ...
Here's what the piston looked like:

Not good. Here's what it did to the back side of the rear right rotor:

And here's the culprit; a little metal plate that rests on the back side of the brake shoe and must have shaken itself loose to wreak pure havoc on my brakes!

Big ups to my ninja C-4 for letting me invade her garage and helping out with the grease-monkeying!!!