If you can move the yoke in and out of the tranny, but not the rear, where's the slop coming from? U-joint straps tight? Did you do the joints or did you have it done? Check the shaft yoke ears, and make sure the holes aren't elongated.. If they are, the steel is stressed and you'll have to have it rebuilt soon or you'll start to eat u joints. That movement is probably not causing the bump, but it's related to what is. Find where the slop is and let us know...
Matt {peace}
Matt {peace}