My 2000 Grand Marquis has a slight vibration on the highway. I do not suspect that the wheels are out of balance as the tires are still new Michelins. I lifted the car and spun the front wheels and noticed that the wheels spins freely 3/4 of the time, and then 1/4 of the time the brakes grind. The wheels gets very hard to turn, the grind is not slight. I checked all the suspension components and there is no play. Can I pretty much be 100% sure that the vibrations are due to warped rotors?
Well, one way to tell about warped rotors is to notice the following: The vibration will be most intense when stopping from high speed. So, they will be vibrating (usually at or near the steering wheel) at one level of intensity, let's say, when you put on the brakes at 35 mph, and another if you're putting on the brakes at 75 mph. That's "a tell."
Now, the only other thing that instantly comes to mind is a brake caliper, or caliper pin that gets routinely stuck. Such "stuckness" usually comes from lack of maintenance.
If there's no play in any other components including the wheel bearing, then yes. 100% warped rotor. Some places will still resurface them if you don't want to go buy new ones but that's a dying field.
If you do resurface the rotors, be sure to also resurface the pads. Otherwise just replace both.
The only other possibility for the pads to be grinding are remote, but possible reasons:
1. The pads were installed incorrectly or have somehow moved from their slots on the calipers;
2. The caliper was bent in some way, and is not straight
3. The pads were out of spec, or even the wrong ones (rare)
Visual inspection would easily determine if any of these conditions had occurred.