Picked mine up late yesterday, and the rocking seat issue has been resolved. Yay! Here are the notes on the work order in case it helps anyone:
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Customer states excessive play in driver's seat.
Tech verified customer's concern.
Tech confirmed and installed new seat track to correct per tech line (#15218003).
After track replacement, tech road tested and found now when seat is going backward and forward it is extremely noisy and vibrating, and again has excessive play. Contacted tech line, was advised to swap motors from original seat track. Tech removed and replaced forward and backward motor, cable, and bracket and re-tested.
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While it was obviously not a world-ending problem, it also wasn't something that I expected on a brand new car with an MSRP of close to $36K. Very happy with the way that both Mazda and the dealership dealt with it.