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It was an option extra. At least on the 81. Which is the only year I have extencive paperwork on.
You could have a truck with no other options except that one.
It seems the most popular options was Ammeter and oil guages without tach, automatic transmission, power steering and brakes, extra and or super cooling, limited slip, dual tanks, etc...
and the most popular packages is one of the towing or camper special packages, that contained some of the above at reduced cost.
well you could always just check it. jack the truck up and spin the rear tires by hand see if the both move in the same direction or look on the door tag for a letter in fron of the 2 digit axle number.