Heck, if you'd have posted, I could have sent you a report after driving "your" truck! I searched carmax too but in the end, decided to buy new but they do have a very good sales system.
The "best deal" I found last Nov was a 2015 Tahoe with 5,000 miles....42 grand out the door at Carmax-Laurel & a '16 Tahoe with 7500 miles at Carmax-Security blvd for 48K out the door.
I'm just posting these to give you a reference.
I have also felt what you describe and have attributed it to the design of the body....if you accelerate very slowly, you can feel it get into it's "resonance zone" where you feel it. Downshift or change physical speed and it goes away....no bad bearings.
In fact, if you crack a rear window, it disappears.....this is a pretty good indicator!
You will love that truck though!! Congrats!
jeff