Thanks for the answers. Just a curiosity of mine anyways.
Like I said, I have only noticed this on Northwest flights. Specifically on the Minneapolis to Bozeman, MT route which is the only route I fly Northwest regulary. I wonder too if there is a higher takeoff speed for Bozeman because of the high elevation and that may make the wheels more prone to cause vibrations once they clear the ground.
Oh well, just curious, thanks for the answers.