If you can rattle it I'd change the bearing. They normally move a little because they only have one bearing to support the shaft, the other end floats in the diff splines. So if you can feel too much play, it will be the outer bearing. That bearing takes a bit of thrust due to the front 1/2 shaft angle. I find drive line vibe is normally related to the front shaft somewhere as long as rear drive shaft coupling is ok.
