It is an “access panel” for the tow hook. Mine is missing as well. I think I am going to use a dremel and round off the edges to make it look more natural and call it good!
This is my first post. Bought a 98 limited 2wd last month. 249,000 and still has life left in her. I would not have bought any other vehicle with that kind of mileage but I got a deal and her body is in remarkable shape to be 20 years old!