16 Silverado shaking badly

I have a 2016 Silverado Z71 LTZ with just over a 100k miles. Earlier this year I had new tires put on it and then more recently I had the brakes replaced. Maybe a week after the brakes were changed the truck would start shaking pretty badly while traveling at highway speeds. I pulled off and checked for a flat tire, but they were fine. I then had the alignment checked and it too was fine. I went to the place I had to brakes done and they said the brakes and the suspension were fine, but they believed I needed a new transmission. $9,000 later I had a new transmission put into the truck after another mechanic agreed that was the problem and it still shakes just as badly. That mechanic felt bad so he went over the truck thoroughly, but couldnt find any problems.

I am still driving this truck with this mystery problem and nobody seems to know what it is. It shakes very badly at freeway speeds and since that is 90% of my driving it just seems that im waiting to be seriously injured or killed (worse yet, kill somebody else) when whatever this is finally fails catastrophically.

Has anyone else had this issue and can give me a direction here? I looked at getting a new car, but nobody has anything right now and ordering one would take months. I need help please.