This is a very simple, 2 minute fix.
On the bottom of the mirror housing on the outside you will see a very small cover. Use a small flat screw driver to open that cover being careful not to scratch anything.
Inside that cover you should find a Torx screw. I don't remember what size it is, but tighten that screw and check to see if that does the trick. Remember to close the cover.
My driver's side mirror vibrated like crazy on my 08 Sport 4x4 DC too, and I found this fix on another website.
Hope that helps.
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