The rear bearing and the front pinion bearing will move in unison because of the spacer pieces, though, yes? My experience has been so...if the front bearing moves, the rear bearing will follow. If the pinion gear is moving in/out with no associated bearing movement, it would mean the pinion nut is loose allowing just the pinion shaft to move or the front bearing is shot and it is not retaining movement as it should. You are correct...that nut has to be tightened until your eyes bug out. I've seen a couple that for one reason or another (and you can't unscramble that egg) were loose.
