The snap ring is to limit axle end play, and admittedly, it is confusing when installing manual hubs if you should keep them or not. The snap ring, on the auto hubs, sits right behind the "fixed cam assembly", so the casual observer may think you don't need them anymore since the fixed cam assembly is tossed. I tossed my snap rings when installing the manual hubs, and never had a problem. I eventually ordered some from courtesy Nissan and replaced them just for the sake of being thorough. Never noticed a difference one way or the other. Can't seem to find the part on their website as it looks like it recently was re-designed since the last time I was there.