Pulsating Brake pedal. HELP!

I have a 2003 Tahoe. About 30,000 miles ago, I replaced front brake pads and rotors, and rear pads only. NOW, I am getting some SLIGHT pulsating when applying the brakes gently. When applying them hard, don't really feel anything.
I have no ABS light on, and when I visually look at my rear rotors they do have some "grooves" or "streaks" going through them. They are not in perfect flush shape.
Is my rear rotors the problem?
Thanks!