Actually it does matter. Not for gearing, but for strength. Marlin recomends putting the 4.70's in the rear case because there is less chance for breakage.
Yes, you can run your front case in low range with the rear in 2wd or 4hi, or any combination you can think of. The advantage to having the 4.70's in one case is it gives you more gearing options/choices along with a lower final gear ratio that can come in very handy.
What kind of 4wheeling are you going to be doing? This will certainly affect what kind of gearing you actually need.
