2003 Ford Explorer VIbration when braking at high speed

2003 Ford Explorer, 4wd, 60K miles.

Vibration existed before brakes replaced. I replaced all 4 wheels pads and rotors. Vibration still exists. It occurs at high speed when braking. It is a vibration, not shimmy (the steering wheel does not go back and forth). The vibration is more noticable at higher speeds. It is not noticable below 40-50 mph. The brake pedal does not vibrate. I cleaned all rotor mounting surfaces to ensure clean fit. I lubricated caliper slide pins. It seems like vibration is more pronounced with light braking at high speed, and seems to diminish with harder braking. It is also more pronunced when high speed braking downhill. Rotors seem to be true. I tested by spinning wheel, while off the ground, with slight pressure on brake pedal. Drag appeared uniform on all 4 wheels.

Thank you.