There are small metal clips that go on the flat spot to either side of the tailgate. On soft tops, it holds the tailgate bar. On Dusters, there's a plastic wing that slips into it (transparent part of the drawing above).
"Tailgate Bar mounting brackets" is (I think) the official term. You can buy direct from us as a replacement part 800-845-3567. Quad and a few others are now stocking replacement parts - and they'll be quicker and probably cheaper.