Please explain cast iron Differentials ?

[Jimbo76]

Lurking here for awhile I have questions about differentials. It seems a locking differential (adventure package) is available for the H3 in cast iron. My question is what are they normally made of and what part of the axle assembly is "cast iron". Is it internal where the ring gear is or is it the whole axle from wheel to wheel that is the desirable part to get ? In the old days we called it a pumpkin. My end desire is to lock front and rear and what is needed to do this. Thanks in advance.