I have got a similar problem with vibration when braking at that speed and last thursday took the car into my father-in-law's garage who is a MOT tester.
Put my 97' GS300 Sport on the rolling road and kept the brake applied to a constant reading as shown on the rolling road dials and my off side front wheel kept a reading of 200 while the near side front fluctuated between 150 and 200 - warped disc. The back wheels were also tested but came out fine.
Will be replacing the discs next week along with pads to cure my problem.