Why should I not tow with a truck bigger than 3/4 ton?

The reason it is stated to not tow with vehicle rated over a 3/4 ton is due to the suspension on the bigger vehicles. They have a much stiffer suspension which put much more stress on the frame of the trailer every time is hits a bump. Over time this could cause the frame of the trailer to crack. The softer suspension of the smaller vehicles help the trailer to ride more smoothly and puts less stress on the the trailer frame.