1997 Subaru Legacy trouble in control

When driving in dry condishions you don't notice much, but living in VT when the roads get snowy it drifts all over the road, it feels like the back wheels are going faster and you have to turn the streering wheel left and right to keep it in the road, this just started about 3 weeks ago as far as I know, just had four new tires installed, driver side rear inside was worn unevenly.