Tires: bad vibration, stock tires, rims and tires

Question
the 275 45 20 is the same tire size as what is stock 265 70 16 same dia. the size never changed and this problem was ther with the stock tires and rims ? what do the rotors and brakes have to do with the vibration its at highway speeds not while braking

Followup To
Question -
i have a 2001 montero xls just put 275 45 20 tires and rims on the vehicle and even before the rims went on there was a bad vibration between 55-80 mph had the wheels balanced 3 times and they don't get any better sometimes worse.considering a roadforce balance any suggestions? need help bad the  shaking is almost uncontrollabe for my wife on the highway
Answer -
Jason
Tires that big are very hard to make. Unless you are willing to pay top dollar for rims and tires.  The rotors and brakes are not set up for this size tire. They are mostly for looks and not highway speeds.

Sorry But I would carry them back to where you bought them.

Answer
The rotors are set up for a 16" inside diameter wheel. when they mount your tires and if they torque the lugnuts at the wrong setting it can cause the tires to vibrate.
without looking at the car and driving I really can't say what is wrong.  I do know the bigger tires 20" and greater do have all sorts of uniformity problems.