I have a 2020 GMC 2500HD Denali with wheel shake / shimmy at 50-80 MPH. It's even worse pulling a trailer, makes the truck feel unstable.
After reading this forum, I took the truck back to the dealer and they said "oh, your tires were slightly unbalanced" so I drove it off the lot and to no avail the same wheel shake, no change.
I then convinced myself I needed new wheels and tires, so I ordered some aftermarket 20" wheels with highly-rated Nitto Terra Grappler G2s with the exact same tire size 275/65R20 (so I could keep the same tire load rating) AND TO NO AVAIL, no change. Still have the shakes.
In my experience, this is obviously not a wheel or tire problem it is a software or hardware problem. The 2020 Denali's steering is "easier" to turn than previous years and feels more like an Escalade so my theory is that they borrowed some steer assist system from the Escalade but it has bugs to work out. There is no way they tested this truck for a year and the techs didn't notice it, unless perhaps the trucks they tested didn't have the steering assist so they never noticed.
Either way for a 80k truck, it should damn near drive itself much less have wheel shake. Go buy a 80k BMW or Mercedes, it will drive it self and while balancing a dime on the wheel. Hopefully GM gets this figured out soon.