A WD hitch doesn't get installed on your truck. It's fits into the receiver of your existing hitch. It contains the ball and a couple of long bars that get attached to the sides of the trailer tongue.
You need a distribution hitch if your load makes the back of the truck sag. It levels the entire rig, distributes the load across both axles of the truck and the axles on your trailer more evenly. It also helps with sway.
A trailer place should be able to give you a good recommendation and will also have the hitches on site for you to see.