The throttle position is still changing although cruise control is set. Any deviation such as wind or grade or just because will force a throttle position change. Granted I suspect it would require noticeable movement before the tach would fluctuate. And speed should be very stable.
Any chance you have electronics that are interfering with the electrical system?
I vote for some sort of electrical problem.
The throttle position can change all it wants. If you are doing 60 MPH steady on the highway and maintaining a steady overdrive gear like 5th or 6th, really should be 6th in the scenario described, then the engine is locked in with the transmission. If tire rotates at a certain RPM, the engine should be rotating at the appropriate RPM given the complete power train gear ratio. Only way that changes is with transmission slippage or gear change.
It could be caused by transmission hunting for a gear, but I think that would really transmit to the whole truck in vibration based on what is being described. I think the gear hunting would catch your attention more than a fluttering tach/speedometer.
Make sure it is reproducible and then take a service tech for a ride and show them. While I would tend to shy away from electronics interfering, unless you have done some major projects to your truck electrically, would put my bets on a loose connection or else a bad BCM or some other black box on the truck. A local service tech figuring out an interference issue will be a long shot. (As they are mostly just parts changers, innovative thinking is not in their toolboxes usually.)
Someone on a different thread suggested that speed is no longer taken from the transmission but from the ABS wheel sensors. Bad sensor, bad connection, etc. in that area? Dirty rotor, broken tooth on the rotor that causes unusual reading?
This seems to be a one of a kind problem, so unless the mechanic has seen it before or GM has seen it on other vehicles and issued a service bulletin, resolving it may be a long diagnostic headache.