While we may get feral hogs in all 50 states, I don't think that they will ever be very prolific in the northern states. While they're fully capable of surviving the temperatures, it takes them a lot of energy to do so. Foraging in the winter is slim pickings in the north. Farmers, ranchers, etc won't want them around, and it will be easier for them to keep them under control than it is in the south.
