Thanks for your quick reply, Steven.
The area with the most noticeable tightness and soreness is on the back of the shoulder; I've tried to indicate the where it is on the picture that should be below here.
Also, I found the movement that seems to irritate it the most, but I'm not sure how to classify it based on the list of articulations you posted. It's the rotational movement that brings the lower arm from pointing down to pointing up, with the upper arm parallel to the floor. I guess its basically the motion involved in a muscle snatch. This video shows it way more clearly than I can explain it:
Thanks so much for your help