Some other questions.

How much are you actually driving the Xterra? How far are your commutes? Is it highway or surface streets? Hills or flat? How comfortable are you with driving on ice? How much traffic?

I have winter tires for my Geo so I drive that more than the Xterra in the winter. I mostly only use the Xterra for short trips around town and stuff. The Geo would handle better on racing slicks than the Xterra would on snow tires, so... Well, maybe that's exaggerated, but the high center of gravity of a truck or SUV combined with the front end weight bias really work against you.

ANyway, even in 4WD I take it real easy in the Xterra and I'm running BFG A/T's. As someone else noted moving forward isn't a problem, it's stopping and turning and sometimes just staying on the road! Even in 4WD the rear wants to slide out pretty readily and the high center of gravity wants to send you into the ditch in corners. But it is perfectly safe if you know the limits and drive with care; ignoring all the other idiots around you that think you're moving too slow!

That said, if you do want to get winter tires you basically have two choices... studded or studless. Personally I prefer studless even though they do not have the same level of ice traction as a studded. I just drive slower! The studless are just a lot nicer if you aren't on ice all the time. In the studded tires it probably doesn't matter much what brand you get or anything, but my neighbor uses a Cooper snow tire of some sort with studs on his GMC pickup and they really work well. If you want to go studless, I highly recommend the Bridgestone Winter Dueler. They don't last long if you drive them on dry pavement, but they are probably the best studless snow tire you can buy for an SUV.

But, if you just need to go short distances and aren't going to be driving a lot on ice, get some BFG A/T's and drive carefully. Get extra siping in them and that will help a bit too. They do a good job for not being a dedicated snow tire.

Main thing is watch out for everyone else!