Adjusted the tension on the RD yesterday, and went for a quick 2 mile ride up the road and back to ensure no issues while shifting. That seemed to be the problem, indeed. When I got back, I just took a rag and wiped excess grease off the rear cogs and bottom portion of the RD with a plain old rag (old XL T-Shirt from my fat days), got it pretty clean - I'll probably use a can of White Lightning Degreaser before I put the bike away for the winter.