Put the car up one corner at a time and grab the tire @ 3 & 9 o'clock: shake the wheel/tire and see if the thing shakes around like it's not bolted in tight. Also try @ 12 & 6 o'clock, too.
Mine was doing that, so I flushed the braking system 6 times and it didn't go away. I even used the Ford IDS system to look at wheel speed sensor signals. All that did was point to the wheel that the axle nut had become loose on... of course I didn't figure it out until parking it and messing with it a week later out of desperation. And when that symptom crept up again, I found out the rear on the opposite side had done the same thing.
The wheel speed sensors use a calibrated air-gap and teeth to do it's "math"; if the axle nut is indeed come loose, it's allowing excessive endplay (a.k.a. too much movement/outside it's calibrated specs) so the ABS kicks on to mitigate what it's perceiving as a traction loss... You may also have a speed sensor issue or broken tone ring. The easiest home-test you can do is put it in the air and shake those wheels to check for endplay!
BTW; my lugs were torqued properly when I did that video.