Are you sure you have a 203 case? I just read your profile and it says you have a 700R4 and 10 bolts front and rear. If I'm not mistaken, a rear 10 bolt and 700R4 weren't an option in any blazer with a 203. Unless of course this isn't that blazer or maybe you have a 208 case? I'm just wondering because I've never seen anybody use an overdrive tranny with an uber-heavy 203 full time transfer case before. /forums/images/graemlins/screwy.gif
If you have a 208 it would be an aluminum case with a 2.62:1 low range and 1:1 hi range. It would also be part time so you would have hubs up front (possibly the auto-locking ones)