vibration during snow driving

My 2000 windstar experiences intermittent shudderring appearing to come from engine area - but this only happens (so far) in the snow. It happens while sitting still and moving, and changing gears from drive to Nuetral to park does not seem to affect it. Since we haven't had too much snow in the past 2 years I tend to ignore it - but it happened to my husband this weekend so he knows I'm not crazy. Could it have something to do with ABS? Haven't discussed it with dealership as yet since they'll never "catch it" and they tend to be vague in their knowledge anyway - this van is fortunately under a 70,000 mile extended warranty but it doesn't help when they can't ID the problems.

I suspect it is the Positive-Traction (or whatever Ford calls it) system on the front end drive system. Do you get a light announcement on the instrument cluster when this happens?

One thing that can cause "confusion" in this system when you are in the snow or slush... is if one front wheel brake caliper is dragging a bit. If you do your own work, I would suggest you open up the front calipers (may be time to replace pads anyway) and thoroughly grease the pins (with dust boots) that the caliper piston assembly slides on. While you are there, thoroughly grease the metal clips at the ends of the pads.
I like NAPA/Permatex synthetic caliper lube ... part #765-2429.

Thanks for the info., I don't do my own work and will probably need brakes within the next 6 months +/-. Is it likely the case that this this is not a dangerous situation for the interim?
I do recall that when this van was approx 2 yrs. old I experienced odd clicking noises from the front end when braking at low speeds. At the time, the service place I use (not the dealership) told me it was the brake clips, not a dangerous situation, and that the next time I had brakes replaced to make sure I got new clips - they had run into it on another Windstar. That problem was taken care of when I had front brakes replaced and as far as I can tell did not reappear.
Thanks again for the insight!!

P.S. There is no indicator light when this ocurs.

Actually Ford recommends that you replace the clips with EACH pad replacement. You may have some bent or missing if you depend on others for service. Wouldn't hurt to go ahead and purchase a set of 8 clips yourself so you will have them when brakes are next done.
And be sure that the serviceperson greases things as I have already outlined.... too often brake service folks will just replace the pads and do no lubrication....except to put some on the back of the pads to stop the squeal...more than this is needed.

I would have guessed the Traction Control system too, since it definitely makes a strange noise - but it wouldn't come on when standing still, and according to bluevan it happens both when moving and sitting still.... So perhaps it is some combination of the Traction Control and ABS that causes the 'Traction control sound' to continue after braking to a stop...?

Dan D