all over the road

Well the wife's car(97 Gtp) has developed a disturbing little trick...under snow/icy conditions when the road is a little rough, the car jogs a little to one side or another, it doesnt slide out of controll or anything, it's like it hunts for it's footing.....it's kind of scary. we have had it to two different tire/garage place ......every thing is tight and aligned, and they don't have any answer other than one rear tire was cupped and recommended 4 new tires. the car currently has Wintermark magnagrip 225-60-16 on it. also the one place recommended changing the bearing/hub assy on f/r passenger side(though they are not loose...just a little noisy) up until this winter the car(with the snow tires) had outstanding manners for a sporty car in the snow....anyone have any clues?

if the toe is out on the rear the car; on slippery road conditions the back end will be all over the place... as the thrust angle of the rear axle will be steering the car in a different direction.

did the shop do a four wheel alignment check? If they only did a 2 wheel no spec for the rear will be accounted for (and they won't know what the rear alignment numbers are). Post the alignment numbers if you have them.

is your tire wear a feather edge wear on the inside of the tire?