I have read on the internet that sometimes they weren't used. I had a 68 rearend that had one on one side but not the other. If it's working fine I think I'd leave it alone.
If you decide you need some be careful ordering them. Some of the ones they sell don't fit because they have a raised lip on the inner ID.
Related thread with source for new ones.
https://67-72chevytrucks.com/vboard/...d.php?t=848797