It seems rather unlikely that both calipers failed in exactly the same way at the same time, or that both flexible brake hoses failed in exactly the same way at the same time. What are the chances? You need to go back to the point at which one failure can cause both rear brakes to lockup. That point is the master cylinder. Simultaneous lockup of the rear brakes is in fact a known failure mode of MB master cylinders from that timeframe. I've been there, done that. Replace the master cylinder and you'll have fixed the problem.
That said, a thorough inspection of the rear brake system is now in order. You probably do need to rebuild the calipers, as the dust boots have likely melted from the heat. The rotors are probably warped, and there may be some collateral damage. You almost certainly need new brake pads.