1991 Mercedes 300E 2.6 - Drifting issues

[bmxkraut]

I purchased this Mercedes 300E a few months back and it is virtually in perfect condition as far as the body and interior. It has a few tiny bugs to work but nothing too serious. The older gentleman that had it put new Blizzack winter tires on it figuring he wasn't goint to drive much before winter, then decided to sell the car since he couldn't drive anymore. It's massive hot here still and all of a sudden while running at higher speeds the car sways when I hit even the smallest bumps. It seems more so from the back than the front. Does this sound more like sway control bars or could it be shocks? Just trying to figure out how much time and money I need to spend? I don't imagine it could be thhe tires..?