I have a 5x10 utility trailer sided with 3/4 plywood that I use throughout the year for various things. (last fall hauling 3 face cords of wood). I also go to lake erie several times a year, 30 minute drive, and I haul my 18 ft inboard outboard boat with it as well.
I never haul in drive, always put it in 3. I am too afraid of putting it in drive on a flat spot and forgetting about it. If your trailer starts to sway, you can put some weight on the tongue to help possibly. (or distribute weight better in thing you are hauling) I would rather waste gas than burn up the transmission.
Some tips:
1) check your transmission levels and color often, and change it every year.
2) make sure your front and back brakes are in good shape, you will wear them down more.
3) make sure your emergency brake works.
4) Don't haul with tires that are bald. Especially at a boat ramp.
Make a test run to see how it works. I am very happy with the tb hauling ability, but I have an EXT, not sure what yours is.