Play in pinion flange

Howdy Folks
Just greasing my u joints and when doing the last one at the rear differential, and noticed a surprising amount of play in the pinion flange (up and down play - not in and out) So the bearing in there is worn. Any one know much play is acceptable - before ring and pinion damage?
Also does any one have experience changing just the outer pinion bearing or do both inner and outer need to be changed at the same time?
Thanks to all who reply.

 

Any indication of gear lube leaking out? Did you check the level of the Duffy fluid level?

If you got plenty of gear oil, then it stands to indicate the pinion seal is still doing it's job.

Can you tighten the pinion nut and remove some of the play?

 

So I checked the fluid level - it was fine and recently changed. Since the rear end has no leaks and and any other issues, I think tightening the pinion nut as suggested above, according to this video; https://www.youtube.com/watch?v=JNFjcx8m7Hk which shows the exact same situation I have. Besides incorrectly over tightening the nut, any down sides to removing the play by doing this?

Thanks to all who reply.