The Dana 18 is found in 1941 -1971 Jeep vehicles. It is also found in IH Scout trucks. It's production spanned 30 successful years, making it one of the most famous and recognizable transfer cases of all time.
The Dana 20 is found in 1962 -1979 Jeep vehicles. It is also found in IH and Ford vehicles as well as a limited run of GM trucks in the early-mid 1970's. The Dana 20's design was outstanding and remains one of the strongest and most refined of all the popular transfer cases.