Vibration after Brake Job

I just replaced both Rotors and Drums on my 97 Tahoe 4x4 (front disc/rear drum). I also replaced the pads but left the shoes cause they still had plenty of life left on them. I did the two rotors and pads this past memorial day (friday). Noticed a big difference in how the pedal felt and even the vibration in the steering wheel was gone. A couple days later (Sunday) I replaced the two drums and cleaned off the shoes as best as I could. The ride and brake feel was excellent for about 15 miles, then after the 15 miles I began to notice the steering wheel would vibrate a lot when I would brake as well as the cab itself. For the most part it vibrates when braking, however there are times when brake at speeds 40 mph and less that the truck doesn't vibrate. I was thinking the wheels need to be rotated and balanced but it only vibrates when braking. Any ideas?

To me it could only be 2 things:

1. out of balanced wheels or bent rim
2. warped rotors

From what you discribe I would put my money on warped rotors, I would return both of them, and get another set. Alot of people have very good luck with Auto Zone rotors.