Hoping to get some details as to what this is that is leaking behind my rear diff. Assume it is maybe some kind of driveshaft bushing, or perhaps a diff mount bushing?

Based on how it looks to get at, assuming this is not going to be a cheap fix. Any ballpark figures?

FUP...took to dealer. Technical term is "Rear subframe mounting bushing". Probably a $10 bushing, but it is not a serviceable part. Entire subframe needs to be replaced. I have an extended service plan that denied it, as the bushing is a wearable part. $4,333.26 to replace

Dealer did give the part to me at cost so that shaved $500, but I was not prepared for that hit. They found my control arms were seized too, fortunately my service plan did cover that. Good news is that the ride home did seem noticeably smoother.

Many bushings like that one are filled with a thick gel , a gel will have have different vibration-defeating properties than like a solid rubber or solid Urethane bushing .
Since you mention extremely high estimates AND I believe that bushing problem is probly leaking gel - my opinion , live with it (Loooong time before develops into more of a "thing" and probly never dangerous) .
The oooze , if bothers you , can wipe off like every month , ya? YMMV
Ask around too , get other's opines too.

p.s. Today's prices ..........I highly doubt the bushing itself is anywhere NEAR as low as $ 10 , notwithstanding whether it is R&R capable or not.

I think I didn't READ this second post..... .....at all ? Temporary dyslexxia or more permanant stupidity ??
scuse me .....so sorry Wow seized ctrl arms too, ya

I go back to youtube and put Bob Schimmal back on
https://www.youtube.com/watch?v=PJRor7_1UwY