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Most exhausts move a little bit (some more than others) and are designed to do that. They are mounted on rubber gromets that keep it 1) from vibrating metal to metal and 2) from making excess noise from that vibration against the frame or suspension components.
That is going to give you a little bit more of a wiggle. You can do some type of new bracket welding job to tighten it up a little bit, but the bottom line is the rubber is always going to be willing to shift a little here or there, causing some wiggle.