But my rear diff pumpkin is bigger than a 1500s (more cooling due to oil capacity) and my 6.0 has a much different torque curve than the 1500's motors, so I don't think I will get failures like Mizzou did.

That said, the 3.73 is why mine can't tow 14k, I'll install 4.10s if I ever burn up a 3.73.

The final drive ratio in the rear end will always determine max torque to the wheels, a 4.10 will always lay down more torque to the road and a 3.43 will always get better mpg. New transmissions with more gears can give you more flexibility, but what I've seen, 1-4 is still power gears and 5+ are over drive and are worthless when towing, in fact locking it at 5th gear on mine is best practice.

Also, 1500s are part of the EPA fuel economy regs and thus are tweaked for mpg not towing reliability. Heavier trucks are exempt (at least they were, they may have changed) and are tuned for work and reliability. My truck gets 12-13mpg empty.....but I trade MPG for stability, durability, and towing reliability.

The biggest issue with a 1500 isn't horsepower and gearing (though gearing plays a role), it's frame strength, rear suspension design, and braking capacity.

It's why a Power Wagon is down rated in towing, because it has an off road rear suspension (it is 4.10s, but softer coils) that can't handle 10k+. IMO, the pure HD trucks, even the gassers, are just a safer towing option above 5k.