They are not sway control bars. They are weight distribution bars. If your Excursion ride without sway I would suggest you leave them home.
Just as you are about to hook up, while on a level surface, measure and mark a height on your front and rear fender straight down through the center of the whee. Place a piece masking tape on the fender and mark a reference point. Now hook up and rear measure the fenders. The rear will drop. It is the front fender you are most interested in. With out WD it will go up but considering the Excursion's front axle weight if it does not go up more than a 1/4 in. you should be OK. If it rises more than that you should consider a WD system. Your trailer and heavy truck would be fine with 600 lbs bars. Heavier bars will just result in a rough ride for the trailer.
I have recently installed an Andersen WD/sway system. The total cost of that system is less than the cost of the correct bars for your current system and provides sway control. You can see my initial comments at post 23 on this tread.