Hopefully someone out there can give some insight. I'm noticing a metallic click from underneath near the rear once I've shifted into reverse or drive. Also from time to time, while starting from a stop already in drive. Does that sound like the yolk or u-joint that's engaging or something worse? Any thoughts are greatly appreciated.

Mine has been doing this for like a decade - I assume it's not detrimental, just more of an annoyance?

Yep, it's just the splines shifting a tiny bit. I've never seen evidence of it hurting anything, even when the nuts back completely off.