I am also seeking advice for this same problem. My incoming water line runs under a large attached ground-level deck plus an addition to the house.
While I haven't yet determined exactly how much needs to be replaced, my feeling is that all the piping under the house probably needs it due to age and internal rust buildup. Part of it was replaced with copper before I bought the house but much galvanized piping is still in service.
I'm open to any information about this replacement problem. I will be hiring a professional to do the work but the one I've talked with so far wasn't too happy to hear about having to dig under the house.