99 f250 PSd shakes when i apply the foot brake

[59beater]

just noticed recently that when i use the brakes going any speed , fast or slow that when i use my brakes i get a shaking feeling or it feels like something is grabbing then letting go then grabbing again, mostly feels like its on the drivers side. had upper and lower ball joints replaced, new axle seals with the ball joints, power steering pump replaced, sway bar bushings and end links replaced, new ford front rotos replaced, calipers replaced within last year, brake pads look good. what else could it be???