Rear Tires Suddenly Lock Up Going 65

I still think its something in the trany. Most likely you will have to totally disassemble to find the problem.
I think that to lock the tires up with the brakes you would have to apply them hard and fast. My duster has 10x2.5 rear drums and optimisticlly 300hp. I can easly out power the rear brakes. You have ruled out the rear end that leaves the trany.