Solved. Thank you all for your input. I learned a lot through this process (now running external BECs on most of my cars). I'm embarrassed to say that the receiver harness on the ESC was transposed....the whole time. So basically I was frying Rxs because the idiot that sold me the ESC had the harness plugged in backward the whole time.

HUGE props to Castle for helping me figure this issue out...even when I bought it used. That's customer service!