Utility trailer that is 5' X 8' ft The axle is 59 inches from the front of the trailer not the coupler You want to extend the back of the trailer 10 inches Would this be safe to do and pull a load?

On a 5' X 8' Utility Trailer you would want a 60% / 40% axle weight distribution for optimum towing/tracking of trailer. A design of 96 total inches in length would produce a 54" length from the front of the trailer to axle center. Adding an additional 10 inches to the rear of the trailer would then produce a optimum 63" length from the front of the trailer. Extending this trailer by 10 inches would fall into a safe range as long as you take into consideration the weight distribution of the load.