Without crawling under my truck, I'm trying to visualize exactly where you are pointing to in the video.
Is that the bottom flange of the bell housing, immediately behind the oil pan?
When I think vacuum leak, I general think intake side, but in looking at pictures I can't find a point where a leak would pull vacuum on the bell housing space, which in turn leads me to think rear main seal, as mentioned in your initial post.
Is there an inspection port on the bell housing, or could you get a borescope in there if you remove the starter? I'm wondering if you could detect evidence of oil leakage around the rear main.
Looking around on other forums, I did find mentions of a leaking rear main seal causing idling issues. In one case it was linked to bad crank case vents.
I think my first course of action might be to change those vents and see if it did any good. as I'm assuming changing a rear main involves a lot of work that i don't want to think about. I'm wondering if you could get the trans far enough back to do it by dropping the transfer case, or if you would have to pull the whole block.