M/T tires are good in deep snow; where they fail miserably is on ice, where you need lots and lots of siping. A/T's aren't too hot on ice either. Neither one has an ideal rubber compound for winter use either.

Without the front wheels being driven, the Xterra is a handful to say the least (I consider ir dangerous!) when the going gets slick. If I were you I'd invest in some dedicated winter tires like the Bridgestone Winter Dueler or even studded tires. Even then it won't be spectacular, but it should at least be safe to drive.

Oh yeah...get the chains too as a backup! I like the SCC brand Z cable chains personally.