I too just purchased my 2021…
I too just purchased my 2021 Silverado, along with the extended warranty. Soon after I drove it off the lot the shuddering began. The day after I took it back to the dealer I stopped in to check with the service rep. He told me that GM had a service advisory telling them to drop the pan, do a double flush on the transmission, change the filter, and flush with Mobile ATF. Oh by the way, it is not covered by the extended warranty because it was a service issue. $890.00 later I dive it off the lot again and so far no issues with shuddering or shifting. Soon after, I read another service bulletin saying at the first sign of subject problem replace the TCM if it did not have a specific part number installed. I took the bulletin to my service rep and still three months later he has not answered my question as to why it was not checked or replaced.