Success!
Dude, you are the freaking man. Thanks for the advice. I actually guessed correctly on the first try and removed the front piece. The whole piece just slightly overlapped the edges of the joint, whereas the back piece sits flush, so I opted to try removing the front piece first since it looked like it would be much easier to get into to pry apart. This is actually really good for everyone to know, I think, in case that joint on anyone else's figures loosens over time. I also didn't even bother gluing it back together, it pegged back in and sits pretty well flush without. Thanks again!
^^^ And then this is my post.