I have an 03 Expedition with almost 160,000 miles. The other day I put it in A4wd, which I don't use much in AZ but was going up to the snow. When I got up to highway speeds, 70MPH or more I noticed a very high speed vibration comming from the front end. I switched back to 2wd and the vibration almost went away completely. It was so little at this point I would not of even noticed.

Any ideas on what to check or what could be wrong? The tires are almost new and so are the front rotors/brakes. I have not had any other problems. If anyone has had this issue and found a fix please let me know.