I have been looking locally for one of these for over a year, and haven't found one yet. It is a filler panel that goes between the tailgate and the rear bumper. They were only on broncos with the rear contour bumper, and only from 1980-1986. If anyone is parting out a bronco, and has a good one, please let me know! Here is a picture of one that SRWillis on FSB has:
1986 Ford F250, Dana 60, Cummins 12V, ZF5
1985 Ford Bronco: Sold