Will take a stab at explaining but must warn that a teacher I'm not. Remember from your old math book that the shortest distance between two points is a straight line? When your wheels are aimed straight ahead the rod between the pitman arm and steering arm (the draglink) is horizontal and level---providing there's been no dropping, jacking up, etc. Now if you hit a bump the axle goes up so the distance between the pitman arm and the steering arm increases but the steel draglink doesn't stretch so your wheel turns as the steering arm is forced to maintain that distance, thus the bump caused the wheel to turn or "steer". A draglink is really dumb so it it the same length (or longer) and in the same position (level) then it doesn't know whether it's hooked to a manual or power steering box. I have an idea that the increased quickness of the power unit made the truck bobble more rather than an increase in bump steer.

Evan