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I had noticed mine getting a good sized dent, put some jb weld in the dent and left a little excess over it for added protection and havent had a problem yet.
Metal. You can verify with a magnet what you have. If you want to repair your existing, I'd suggest you do some gentle tapping with a hammer and increase the size of the dent to provide more clearance before you fix the hole. Otherwise visit the local wrecking yard with a magnet looking for a newer tube.
Both my and my dad's truck have the dent, no holes yet.
Its kinda funny, I think I posted about this on ... Monday, asking what it was called so I could look for the right replacement part in the yard.