4 Wheel Drive/SUVs: 2002 F150 XLT FX4 issue, rear rotors, wheel bearings

Question
I just bought a 2002 F150 XLT FX4.

Under normal braking, everything is fine.

However, when I am at speeds of say 55+MPH and I brake a littler harder than normal (nothing extreme just harder than normal), then I get a shimmy in the steering wheel until I get back below around 45MPH.

Is this a warped rotor?  More likely the front?  Would it make sense that I don't get this shimmy other times if it is a warped rotor?

Or is this possibly another problem?  Worn wheel bearings?  Bad idler arm?  Ball joint?  ABS?  Other?

Answer
The problem is most likely in the rotors , you can rule out the rear rotors by doing this, step down on the e brake a few clicks and than brake if the shimmy is still there it is the front causing the problem , if it goes away it would be the rear rotors that need replaced , let me know if I can further halp thanks and have a great day ADAM