I had the same exact issue and my dealer is installing a new exhaust system in 7 to 10 days when the parts come in. Glad I found this sticky on the forum
as the dealer had no clue what was causing my vibration.
My dealer would not agree to swap with a GMPP exhaust or any aftermarket exhausts for me either.
Anyone know if they've actually improved the OEM exhaust components so the issue doesn't happen again?
Just wondering if I'm going to have the issue re-emerge in 6 to 7K miles on the replacement exhaust too.