Car wobbles and slides side to side over manhole covers & different road surfaces

kfull

So whenever I drive over manhole covers, specifically ones that sit slightly lower than the street, when my rear tires ride over them, the entire car slides left and right very noticeably. This also happens when going over potholes, or different road surfaces. It was even more pronounced recently when it was raining out. I had alignment done a few months ago and they did mention that the right rear adjuster was maxed out, not sure if this could contribute to it. It almost feels like the car is trying to fishtail when I go over these manholes or dips in the road. Bumps in the road don't seem to cause this, only dips or manholes. Is this simply an alignment problem or could it be a major repair job? I'm bringing the car into the shop on Saturday to get it checked out.

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2012 GM 6MT sports package | FI non res cf exhaust | K&N Drop Ins | Viper 3303 security system w/ tilt sensor | GTR start button | 35% rear 45% side window tint | Michelin PSS