Update on my truck. I met the service director this morning and he drove my truck with me in the passenger seat. He acknowledged a vibration at speed and thought it was "harmonic" in the drive train somewhere. He put it in neutral a few times at speed and noted more vibration when in gear.
He did not rule out tires or rims but the RFB balancer is not operational at this time.
When we got back to the dealership, he mentioned swapping out a set of tires and rims from another truck on the lot but wanted to drive another one to see if it had same vibration.
He also said it did not feel like a driveshaft, but more of an "everything" issue, meaning transmission, exhaust, and engine related.
The bad news was he told me up front GM Tech may need to be called because if it is said combination, it will be very hard to diagnose.
I won't know the results of his driving another truck (although I did drive one and it wasn't as bad) and a recommended course of action until after the holiday weekend.
I had a friend who worked at a GM dealership for 5 years drive the truck and he noted same vibration at nearly all speeds and was kind of perplexed by it. He is going to talk to some of his tech friends that work at larger dealerships to see if they have dealt with this issue.
I drove his LT double cab Sierra and it drove like a truck should.
I guess I will see what next week brings after the holiday.
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