Rear Tires Suddenly Lock Up Going 65
I find a situation involving brake pressure difficult to believe.
In order to cause this, you would have had to just about "dynamite" the brakes to cause that much pressure. Even a fairly hard stop would not cause this, and you would have remembered that at the time.
This SURELY has to be a mechanical problem in the axle or trans.
If you have a limited slip (sure grip) I could see where a brake mechanical hardware problem to be viable.
YOU NEED to tear into this and find the problem.