Rear Tires Suddenly Lock Up Going 65

I don't believe a hydraulic issue is the problem either. Nor do I believe something in the valve body could suddenly do that. Sounds to me like a mechanical issue and I wouldn't be one bit surprised if it is something in the rearend although what Kim mentioned about the output shaft support freezing up is also possible but generally if something like that swells up and sticks it doesn't release and work fine afterwards. For it to lock the wheels so violently that it skidded to a stop it has to be a mechanical issue. I have personally had brake lines go bad that locked the wheel up but it never did it on it's own. When I stepped on the brakes it didn't release and even then it never skidded to a stop. I have also had a master cylinder problem where it didn't release when you let off the brakes but again it only happened when I stepped on the brake that it didn't release. You said you didn't touch the brakes when it happened so how could that possibly be the cause? Hydraulic pressure doesn't just happen. It's the result of you stepping on the brake pedal that pushes the m/cyl. piston in to create the hydraulic pressure.

First thing I'd do is jack the car up and crawl under and check the rearend to see if the pinion has slop in it.