Sometimes sealing around a pipe like that will worsen the situation - whatever heat was getting into the wall cavity is now sealed out. Same goes for insulation. You're right that it's unusual for a 2" drain pipe to freeze. There's probably a clog building that has been exacerbated by freezing. Liquid plumber might generate some heat and open it up. However you do it, it sounds like you have an obstruction that needs to be addressed.