5.8 and 8k on the mileage
You mean 5.3L?
Must not be broke in yet, drive it another 92,001 and then we'll be able to fix it out of warranty.
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Sorry for the sarcasm rcroff, seems GM is having all kinds of shifting and vibration issues with the 2014 - 2015 Silverado's & Sierra's.
Based on other owners; resolve is few and far between, it frustrates us all, not nearly as much as it frustrates you, but many on this board love to apologize for GM, they simply think you and your problem doesn't exist, 8,000 miles should be way more than enough for your truck to "learn" and "break-in".
I recommend taking it to your dealer and tell them exactly what it’s doing, you might get lucky, they might resolve your problem, either-way report back what they do and if it helped, you corrective action may help others.
Best of Luck,
Ed