I think you must remove the black plastic cap on the bumper. Use a drift to punch out the center of the plastic rivets on each end in the wheel well, then go under the truck and work the retainers out through the metal bumper. A special pop rivet tool is required to set the new rivets when reinstalling the cap. New rivets are available at the dealer; they might let you use the tool if you have a good relationship with them. Body shops also should have the tool. Rivets are also available at auto supply stores. Take one of the old ones with you. Marson makes the plastic rivets.
I don't know if it's possible to get at the adjusters using a screwdriver with a flexible shank, but you might explore that possibility. I have just installed a fog light kit on my truck, but I don't have the bumper bolted back in place yet.
Tom