I have the EXACT same problem with mine as well. I took it back to the dealership to have it checked, and only thing the could find was the tires needed rebalancing. Didn't really help at all.
Mine is a `18 CC diesel 4x4 with 7500 miles. Problem has been there since I bought it. Tires are Firestone Transforce AT. I have noticed on mine that it's road surface dependent and usually sets in somewhere around 69-72 MPH. I think the tires are the cuplrit, but not ready to drop a grand on a new set to test a hunch.
It seems odd, but it feels worse to me when the driver's window is rolled down. I think that vehicles are designed now that any disruption in airflow will produce significant wind buffeting.
I played with the air pressure prior to taking it in, but didn't change things.