Generally you are correct, in this case not quite as critical. Cast is like steel in that there are dozens upon dozens of different alloys or mixtures that achieve the desired strength, elongation, and hardness one is after. In our world, we like the high strength nodular iron and the cast in the stock TJ Dana 44 center section is very close to that. It requires very little to prep it for this type of welding. Basically get it shiny clean, heat it up slightly to drive out any moisture, crank the Mig up a bit higher and run a hot fast pass, then back down to normal and do a cover pass over the top of that. Cover it with a welding blanket or piece of fiberglass insulation to keep any breeze off of it that will fast cool it and it's good to go.Hardfacing wire is used to put a very hard and abrasion-resistant surface on an item. It's used on excavation equipment all the time. I assume mrblaine is recommending that because it is very resistant to sliding on rocks, not because it makes welding wrought steel to cast steel easier. Welding wrought to cast requires pre-heating and controlled cooling because the two materials expand at different rates as they cool and will crack if not done properly. I think a high silicone wire will help with that, but it takes some skill to do it right either way.
OTOH, the crap iron that is in a Ford 8.8 is so challenging to weld using all the high zoot specialized space age alloy electrode, heat, cool, peen, stand on one leg, twirl while whistling the Ode to Fancy Welding anthem off key while wearing a sombrero and it still fails, that we won't even try any more. It is a crap axle anyway, why bother?