My 2018 3.7 was shuddering intermittently at about 45 MPH. Called the Chevy dealership where I purchased the truck, and they said it would have to throw a code before they’d do anything. Called another dealership and they were clueless saying it only effected Cadillac models. So called a third Chevy dealer and before I even finished my sermon on the TSB etc. the service writer stopped me short and said to bring it in and they would do the flush no problem.
Dropped it off this past Saturday and picked it up Monday morning. 100 miles later it noticeably shifts much smoother, and after about 20 attempts, no low speed shudder experienced this week.......
Keep calling until you find a dealer who knows what they’re talking about. Post up your city here and I’d bet theres a forum member in your city that can recommend a dealership that worked on their truck on the issue....