They are not too rare. I have a '78 F-250 2WD with the "Trailer Special" package. The package consisted of heavy rear suspension, rear wireing for trailer brakes, X-tra cooling radiator, additional trans cooler, bigger mirrors, dual piston front calipers, wider rear drums, wireing under dash for brake controller, and other little doo dads.
If I remember right the badge on the tailgate was in the shape of the continental U.S. or a rectangle, not sure though.
It was avaliable on Bronco, 150, 250 and 350