Numerous trips for my truck for the vibration/shake. 2016 LT Z71 with 5448 miles. My drop down armrest/console would shake over an inch side to side between 70 and 75 at times.
1st trip ... road force balance, then a run with the pico scope. Readings were 27, 4, 48 and 8. Replaced 2 tires and one rear shock that had a leak. The arm rest shake was gone but a slight vibration in the steering wheel was present. Drove it 1000 miles and no better.
2nd trip .... run with pico scope showed right front out of balance. Road forced tire. Same feel in steering wheel up to 75 and then a bad vibration.
3rd trip ... Dealer contacted GM and was told to replace steering bushings. Was told GM came up with a liquid filled bushing to help with the problem. It took care of the vibration at 75 mph but after 600 miles, slight feel in steering wheel persists.
4th trip ... took a drive with shop manager and another guy that travels to the multiple locations of this dealer and 'trouble shoots'. He felt it and used the word 'pecking' to describe it. Both admitted GM has a problem and they don't have a fix for it. Took a cue from dmetz82 and took a half full bottle of water with me. The shop manager in the back set zeroed in on it but of course nothing severe showed on the road. They decided to run the pico scope again, attached to seat frame and steering column to get a reading and contact GM for a recommendation if needed. Shop manager called and said the truck was in spec so nothing else they could do. Asked what 'spec' is, and was told 15 and was provided copies of the readings.
SO, that is where I'm at. On and off slight vibration mostly at highway speed. Sometimes I think I feel it at slower speeds but the shocks on the Z71 transmit everything little thing on the road. I am surprised service didn't bring that up as a contributing factor for the feeling.