It varies from locale to locale and what type of terrain you plan on venturing out on. Though I must first say, that unless your very peticular about keeping your car washed in the winter time, salt is horrible for your nice premium Jaguar. Anyhoo...If your XJR came with the all-weather package it should have all-heated seats and traction control. The traction control does do its job, however the car is still RWD and isn't the best option under very "tough" winter conditions. Such as, steep hills going up or down, or very twisty roads with large patches of black ice etc. It'll do fine going in a straight line and manageable conditions, but in my experience, anything heavier than that is a flip of the coin. I however have only all-weather tires, so you may have another advantage there too.

 

I have a 98 xjr in Indiana. Winters here are pretty hard core, and I'm very impressed with the way it handles in the snow for a RWD sedan. If you make the plunge, I recommend having a set of summer tires, and good set of winter tires - sounds like you're all set there. My winter tires rock (pirelli). I had the car briefly in the winter with a pair of "all-season" tires, and it was a bit rough - but just fine with good winter tires.