Mine steering wheel has a slight "shaky" feeling above 80mph too. It's nothing to worry about as long as it doesn't get worse or is a very pronounced shaking.
If your steering wheel is shaking so bad it's hard to hold onto, that's a problem.
yea because you are running different size tires on a different size wheel.
This happened to my brothers Tundra when he got some big tires with Ivan Stewart rims.