Hmmm,
Well, if alignment checks out, and if your tires are round and the suspension is set correctly, and you only feel the shudder above 65, then it probably isn't brakes, or steering assembly. That you would feel at any speed, especially if you are braking. My guess is bearings. My dads cadillac used to 'hobble' at speeds over 65. Make sure your wheel bearings are in ok shape. Dirt, water, corrosion, a funny bump, or just old age could make it act funny when asked to spin fast.
Seth