Thanks for the replies everyone, and Merry Christmas!
Hmmm, I think I found the problem!
I ended up making it to Home Depot 1hr. before closing, and picked up the last Ridgid Power spin that they had on the shelf:
It has the option to attach to a cordless drill, but I just used the hand crank. It pulled out some hair, but mostly it was blocked up with black slime - the same stuff that I found in the Nipples that I replaced. I'm assuming that this is from the galvanized pipe? Not sure. It works just fine now.
I'm planning on opening up the wall in this area anyway, so I was going to try to replace this small section with PVC - depending on how it attaches to the main behind the wall. When I get to that point, I will post a new topic. Thanks again everyone!