The nut in the center of the yoke obviously holds the yoke on but, it also adjusts the preload on the pinion bearing. If/when the nut is loosened or removed, the pinion shaft will move up and down. Before removing the yoke nut, you need to check the torque necessary to rotate the pinion with an inch pound torque wrench and record for reassembly. Then mark the pinion shaft and yoke with an awl. When reinstalling, tighten the nut until the lines match up, then check rotating torque with an inch pound torque wrench and adjust as necessary. If the torque is not correct, over time it will destroy the pinion bearing.