Normally noise is your first indicator of a differential bearing issue. In most cases if the is a lot of metal on the magnet there is a good change all the bearings will need done. Seeing minor damage on the internal bearing is tuff. If you decide to take it to a mechanic make sure its one that has done GM rears in the past. Weather to have it done by a professional with experience or you get a complete rear from the yard is a matter of economics. A good mechanic will warranty his work. A yard will just give you a refund or another rear, that is a redo on your part.

There is a process for replacing the pinion seal that does not involve removing the differential. Check youtube. I am not sure what you mean by input seal.

 

@Gorn the seals at both ends of the driveshaft are leaking,
At the input shaft, where it connects to the transfer case at the front, and the pinion seal at the differential in the rear.
I don't have any differential noise, I'm not sure how we got to that topic.
My noise issue is a bad wheel/axle bearing on rear passenger side. I have to drain the diff to change both axle bearings and seals.
What I was questioning was if the diff should be drained for the pinion seal replacement.
It would be counter-productive to drain the diff, fix the axles, refill the diff...then a week later, pull the driveshaft to replace the seals, only to find that I need to drain the diff again for the pinion seal. I'm only asking because I cannot physically do the axles and the driveshaft seals on the same day.

 

 

You can change the pinion seal while you have it drained. and if you have the drive shaft out you might as well do the slip yoke seal in the back of the transmission.

 

@Gorn Awesome! Thanks, very helpful video.