You're experiencing a problem called "bump steer". If the drag link between the steering gear Pitman arm and the spindle is not parallel to the ground, every time the axle moves up or down the wheel will follow. Has the ride height been changed in addition to the steering gear swap? With the truck at its normal running height and the wheels pointed straight ahead, the Pitman arm and the drag link should form a 90 degree angle, and the drag link should be level. That will minimize bump steer through a longer vertical travel distance for the axle than any other position of the mating parts.
Jerry

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