I experienced this rear wagging a couple of times this year. 07 OB XT (manual) with VDC - winter tires are Dunlop WS M3. This year's winter weather has given us lots of icy road conditions versus our typical packed snow conditions. On the road to the local ski hill, one I have driven 100s of times each winter for decades, I have had the rear of the car wag back and forth while driving at a constant speed in a straight line. It is so unnerving I slow way down and have pretty much every other vehicle pass me. I also had this happen recently while driving I-90/I-25 in Wyoming. The roads were horrible, probably the worst I have ever experienced. At one point the car drifted to the left, into a very strong side wind, and I was the by far the slowest traveling vehicle on the road at about 40mph.
In nearly all other snow, packed snow, snow/ice mix conditions the car is rock solid. I am a very experienced winter mountain driver. This is the craziest feeling. VDC on/off does not seem to matter. It seems to happen on relatively consistent icy road conditions at speeds of 40-55 mph when going in a straight line. Folks are welcome to challenge my driving, but I can assure you, that was not the issue. Something in how this car transfers torque from side to side in these kinds of conditions seems to resonate and translate into side to side movement of the rear of the car. I have been in similar conditions, even recently, in other vehicles where this has not happened, even at higher speeds. For example, my 97 4runner did not have this problem on the same road on the same day this year.