Rear Tires Suddenly Lock Up Going 65

The cracked brake shoe material seem to be the clue here.
Either fluid not returning to the master cyl or overadjusted brakes, as they get cracks in them when they get overheated.

Let's see.
My original post on this was heated brake lines causing a hydraulic lock, and overadjusted brakes would make it even more pronounced.

I'd bet you have a high and tight brake pedal, no?