Rear Tires Suddenly Lock Up Going 65

Take a look at the rubber hose going to the rear brakes. If a partial collapse happens, the brake fluid is not getting back to MC as quick and the rear brakes will be partially "on". Doesn't take long and at 65 the shoes will warm up and grab the drum (not sure how tight you have your rear brakes adjusted). When you pull rear brakes everything will look fine. Try jacking up rear axle, put stands under axle and block front wheels. Put in gear and apply and release brakes while giving little gas (you cannot spin wheels by hand fast enough to generate enough friction (heat). See if this will cause brakes to slowly apply.