2 speed rear end

Hey just wondering, my dad was telling me about a two speed rear end that was in an old rambler he rode in one time that acted like an overdrive, anyone know anything about this? im thinking about getting one of these for my 64 valiant because apparently someone swapped the rear end on my car at some point in time and now the car has a hard time gettin up to 65 mph. (just in case you need to know its a 64 valiant with a 273 v8 and the torqueflight 3speed push button auto) any info woud be great thanks guys