I had a similar problem with a new Y2K dually. The dealer installed nerf bars as I requested on the new truck before delivery. They squeaked and I thought it was in the suspension. I took it back to the dealer and the tech and I checked everything on the suspension and found nothing. I took the truck home and kept checking and finally discovered it was the nerf bars. I loosened up each nerf bar mounting bolt one at the time thinking it would relieve stresses and re-tightened them very tight with my 1/2" drive air impact wrench. The nerf bars never squeaked again.
Bill