It isn't just the weight rating on the truck the single wheels just doesn't have that much tire contact patch and dual wheels has a lot more staibility the single wheel the side wall of the tire isn't really strong enough and it wants to wobble with you .
I have towed about with everything out being from the car business also the dually will work a lot better for you than a single wheel truck you can tell the difference very easy when you start out on the road.