Hello, Mike,
not nice to have that problem. Unfortunately you might have to look some further since a lot of these symptoms are mimicked (e.g. T.O.S)and diagnosed with long thoracic nerve injury. Personally I would lokk into your posture. I am pretty sure you have protracted shoulders thus limiting the movement in general of the shoulder girdle and thus asking for problems. Also protraction of the head and neck symptoms would suggest other problems contributing to the problems you sense.
So; 1 be sure about the diagnose. 2 Get your posture right (tight pecs? I'm pretty sure because majority of army or ex army people like to work out in the gym and creating a disbalance between the muscle groups which are responsible for force and other groups which are responsible for positioning joints). 3 Get a back pack which is good for you thus the right size, lying close to your back. tighten the shoulder straps and strap between the 2 shoulder straps enough while avoiding to protract shoulders. 4 get a good movement and postural assessment!!!!