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I just had the front mono-spring replaced on my '04 because the old one was spliting/separating. When I drove the car home, there was a noticable vibration in the steering wheel, a cyclic chassis vibration that I could feel in by butt and feet, and a occasional thud/bump/bang that sounded like it was coming from under the floor just behind the passenger seat. The GM dealer that installed the spring has the car again, denying the feel of any unusual vibration and claiming the noise is from the rear suspension, which they didn't touch. WTF!
They probably have something off in the front end alignment. That can make the car feel like A$$.
And they likely took apart things that affect alignment when replacing the front spring.
If the rear is straight and the front is not, then the rear can try to go a different direction, making it seem like the rear is the problem (I would think).
I'd get it away from the dealer monkeys and find a good alignment shop.
If something is off, they can tell you (front or rear). When you find out the cause, then you MAY be able to blame the dealer, but they'll never admit anything if you can't pin them down.