1999 Ford F-150 Bounce during high speed braking

When I brake my front end bounces when I am at freeway speeds. The steering wheel does not turn side to side, it just vibrates. The truck does not pull to either side when this happens. I've let go of the wheel when this happens and it drives perfectly staight. I did three things just before this symptom occured. I changed the disk brakes on all 4 wheels, I took the truck off rode and there were a few times the front end bottomed out in the dessert dirt, and my ABS warning light comes on once in awhile. Could you describe to me what might be happening, and what I should check out to fix the problem.