What is causing vibration in 2017 Ford Explorer?

What is your question?

Vibration is felt only for a few miles of driving after vehicle has not been driven for awhile. The longer the vehicle sits, the worse the vibration. Vibration can be felt through floor, seat, and especially the steering wheel. After driving for a few miles (5-6) the vibration goes away completely. Vibration was there from day one (vehicle has approx. 30k) and dealer claims to have no idea what might be causing it.

What makes this problem better or worse?

Letting the vehicle sit for long periods of time.

How long has your 2017 Ford Explorer had this problem?

Since day one