That's a problem you'll always run into and the more stuff you add to your truck the more you'll run into it. An aftermarket manufacturer can't possibly take into account every aftermarket item out there and design their products to be compatible with them all. The most you can hope for is that they did a good job of making sure that it will be compatible with all of the factory options. I've often had to modify aftermarket products to fit because of some other aftermarket product I've added. It's the price you pay.
A great way to deal with that btw is to post it on here if something you are going to install is in the same rough area as something else. Like in this case you could have put up a post with the subject "Any problems with installing a hellwig rear swaybar with a pullrite under bed hitch on an '04. 5?" Most likely someone here has done whatever install you are looking into and can tell you any problems you might run into. I've seen that done here MANY times and have always seen results.