Thereās nothing you can add post factory that will increase the towing capacity of a vehicle without having it recertified (which, from my understanding is difficult and expensive). If Ford eventually created some HD towing package from the factory that had higher ratings, you could tow within...
I've owned numerous vehicles, and towed with them as well. Nailing down limiting factors isn't always as easy as you make it sound. I owned a heavy duty payload package F-150 that had a tow capacity similar to my F-250, but with a much higher payload capacity. The 150 towed like crap, even...
And is unibody vs body on frame
Hmmmm... wonder what it is then? Defender is basically a unibody... I would think a truck chassis would be better for towing.