I probably do have some muscle imbalance. I can't raise my arm to the side or do any external rotation without intense sharp pain; internal rotation is also painful (but less so) with some resistance. Sideways plank (resting on knees) is impossible until I find a good spot, and just getting into that position is painful. I'm not a "pain whimp" but I'm afraid to push things until I know what's going on.
Again, without examining you in person it's hard to tell, but it sounds like this may be a disk problem. A neck MRI will clear that up quickly. The neck surgery is a pain, but it fixes it. I was able to go back to racing MX after my surgery, then all 5 lumbar disks blew. When the low back is gone, so was the motorcycle riding, but at least I can ride a road bicycle (but not a mountain bike).