It's probably the front rotors but if you have drum brakes in the rear, pull on the handbrake at 45 mph. make sure to keep the button on the handbrake pushed in and don't lock up the rear. If you feel it in the handle then you know it could be the rear. But it could also be both.
I agree, the same exact thing happened to me. It would pulsate as a brake around 45 MPH, but I got the brakes rotors replaced. They are good as new , no more pulsation. I think it shouldn't cost you any more then $100.