Probably worth bearing in mind that you will have to deal with snow most of the year in MT/ID/WY. Depending on where you live, you may not need 4WD/AWD. Depending on where you live, it might be nice - or it might be necessary. Cuz when it isn't snow, it's often either mud or ice.
Where I'm at, 2WD will get you most places you might want or need to go (there are plenty of trailheads you can reach with 2WD, but they're going to be closer to town and therefore busier). But it won't get you some of the places you might want to go, especially if you want to go into the mountains far from other people. I would advise bringing or buying skis or snow shoes, cuz there are lots of places where you won't be able to get anywhere on foot without them.

