GMC Troubleshooting Questions? Ask a Mechanic for Answers
There is a possibility that this is a torque converter clutch shudder. Your mechanic can test this by taking it for a road test and replicating the vibration, then using a scan tool, simply disable the TCC and see if the shake goes away.
If there was anything else in your transmission that was slipping and giving that sort of vibration, you wouldn't have clean fluid and your transmission wouldn't last very long. If it's in the transmission, it's the TCC. Otherwise, it's somewhere else.
It is easy enough to pinpoint, he shouldn't do anything with your torque converter if he doesn't take the time to verify that disabling the TCC makes the problem go away.
It can also be a driveline vibration, but that has a distinct feel. He/You could just remove the front driveline altogether for purposes of road testing to confirm. As for the rear driveline, I would at least remove it and manipulate all of the universal joints to be sure one of them is not binding. You can't verify this without removing the shaft, it's very evident with the shaft out of the truck.