The bumpers have a little hole in the middle of them, into which you insert a 3mm allen key. Lefty loosey, righty tighty. Slacken the 3mm allen off, slam the tailgate, then retighten.
My tailgate was pretty bad too. The bumper wasn't even touching the body on the passenger side. I played around with the hinges and bumpers and it's pretty good now. Just needs moving over to the driver's side a couple of mm to get the alignment perfect, but that isn't possible without modifying the hinges.