The Dana 20 was used in CJs from 1972 until 1979. It was introduced in larger Jeep trucks in the 1963, but it wasn't used in any Jeep trucks after 1979.
Identification: The Dana 20 can be identified by the center rear output, cast iron tail housing, and deep oil pan. The Dana 18 has an offset rear output, other than that, it looks a lot like a Dana 20.