dear dekkly

it sounds like a torn muscle from your description...to be sure resisted tests will need to be done....take ur arms away from your side...bend your elbows, keeping your back straight push your elbows against a wall, what happens? (almost like you waant your shoulder blades to clap but you resisting the movement with your elbows)
secondly, while sitting, rest your hands on the arm chair and lift up like you would do a reverse dip, do once, what happens? (almost like you want to bring your shoulder blades down,)
thirdly, place one hand behind your head and resist taking your head back with your hand, what happens?
if your wife is there, ask her to place her hands on your shoulders, tell her to resist your while you try to lift your shoulders up (like in a shrug?) what happens?
Do you get pain on full range rotation?
if so...fold your arms acros your chest, turn left just before the pain increases...ask your wife to ressist this movement, what happens?
do the same for the right? what happens

my thoughts again:
I am excluding a subluxed rib hypothesis because you are not getting this pain when you expand your rib cage, you are getting your pain mainly when you move your spine, when you elongate it and when you turn it...as a variety of muscles cross from neck to spine, from spine to scapular, and within spine...
we are resisting these movements to see which aggravates...

if when your spine is stable and the resistance tests are positive..i.e keeping your back straight and pushing elbows against wall, resisted shrugging etc...then its probably a muscular problem...
if its not and you only get the pain on end range of spinal movements, I would put my money on a torn ligament in one of the verterbrae...

However it is essential that you get an xray to make sure nothing is out of place...

Your therapist maybe right to think it is posture related as if you have a faulty posture, i.e a forward neck, you could have a rigid neck/chest junction...with regards the possibility of you having arthritis...spinal dysfunction may become evident...

reply and we go on with our reasoning...

cheers