After landing or during a high speed taxi, I feel a severe vibration throught the entire plane. Enough you can see the wing-tips shake. I thought it was flat spots on the tire, but after putting new tires on, the vibration still continues. I typically feel it between 20 & 40mph. This is NOT a nose-wheel shimmy. It's definetly in the mains. Could it be the scissors? Appreciate any advice you have.