2001 Buick Lesabre Vibration in the front wheel when brakin

I recently installed new brake pads which I have done many time on my other cars but was first time on Buick. At high speed when braking it vibrates and this does not happen at speed below 40 mph. I did not change the roters eventhough they where slightly worn out and did not think that need to be changed. If it is the roters than I neee help from you in detail how can I do this and where I can get digrams of seeing how to do it. Also what advice you can give on either putting new rotors or have them taken to shop for reserfacing. All the help will be highly appricated. Thanks