2" line is required for a shower. 1-1/2" line for a tub. So, it may have been plumbed for a shower instead of a tub. Not a big problem, but yes, you need to probably tear out a little bit to see what's there, and you'll need a fitting to make the sizes match up. It would be rare that the pipe was exactly where you wanted it, so tearing out a little will allow you to accommodate the tub drain. You need to verify there's a trap in there, and after you get things checked out, you might go with a 1-1/2" trap - it might give you a little more leaway verses the bigger one.
No need for anything on the walls although you may want to put up a vapor barrier. Most of those that I've seen are designed to bring the drywall down over the flange of the enclosure.