You can find out if the vibration is coming from the front or back wheels with a simple test. You need to do this safely, so find a road that's not busy, or, better yet, a parking lot where you can safely drive at speed. While driving the vehicle at the speed that the brake vibration normally occurs, activate the parking brake or emergency brake. Don’t fully apply it. You only need to apply it a small amount to get a result. When you do this, you isolate the rear wheels from the brake system. Most parking brakes lock the rear wheels using the rear brakes. When you activate the parking brake, you're often working the same components in the rear brakes that are activated when you depress the brake pedal, but during your test, they are the only brakes being applied. So, if you have a vibration when doing this test, you have warped rear drums or rotors. If you don’t feel the vibration, the problem is likely with the front brakes. Here's a video about the specifics of the test.
As stated in the video, this test won't work for all vehicles. It depends on the type of parking brake setup you have. See the Brakes article for more details on this topic.