If you buy your Ford 9" built from Currie Enterprises one of the options is a welded on skid plate. Well a little known fact is that you can buy this skid plate separately directly from Currie without them building you an axle or performing the labor. And this route is surprisingly affordable. The skid comes pre bent and ready for you to weld it on.
As with everything fabrication and welding related, prep is key. Be sure that you clean the area to be welded completely prior to welding. And as always proper safety precautions should always be followed when working with metal.