I picked my truck up today. The Servive Manager tells me what the engineer did.
He used some device to read the vibration on his test drive. He replaced the following parts,
1) Exhaust System
2) Placed 2 clamps on the driveshaft
3) Replaced the coupler connecting the upper and lower steering shaft
4) checked the driveline angle(within specs)
He tells me the engineer says there is a 50% improvement in the vibration.
We go for a test drive. I accelerate up to the target speed zone (41-45). Absolutely no change, maybe a bit worse.
While the truck is vibrating, I tell the SM to put his hand on the steering wheel and airbag. He says, "I feel the road coming through the tires." I say but it is a constant vibration and not just bumps. He would never admit to feeling the vibration. The truck would even start a low pitched hum when it was vibrating and he never heard it. GM can't deny that there is a problem because the engineer threw all kinds of parts at it. I guess I'll take it to the next level.