shimmy under braking

when i apply the brakes i get a shimmy in the rear between 45 down to about 35 then goes away.
doesn't shake above 45 or below 35
2001 supercrew 4x4
any ideas?

check condition of your rotors you could have one warped or fords are known for the surface of the rotor flaking off because of heat then look at all your sway bar bushings