Well I don't think the step bars would squeak, they could creek as they are just bolted on. But if it's a rubber type squeak it the front suspension. Otherwise just tighten down the bolts on the step bar, and that might do it.
Best thing to do if it really bugs you is to have a friend step up on the step bar, and try to get the sound to occur over and over. You would be trying to locate the source of it while it squeaks away.
Hope that helps.