I've had people joke that my left shoulder issue is being caused by my right big toe, which was always a possibility; that guy doesn't seem trustworthy...
Last night, though, a new culprit surfaces; a random spot on my right rib cage (this is how good at anatomy I am).
While testing my overhead reach on the left, I get a cramp on the right ribcage. Muscle to be named later. After two minutes of torturous poking into that spot, suddenly my left reach is better and I'm stronger on that side, too.
A miracle? No, although that the cramp appeared, revealing the area that's 'causing' the problem might be.
More to come; more testing and torture, tonight.