Your problem is most likely aggrevated by riding position so you should watch for a few things:
Do you sit straight up or slightly leaned forward?
If you are leaned forward and rest your left hand a lot while keeping your right hand on the throttle you may be slightly twisted.
Does your head bob at all from the wind:
If so you're involuntarily working your neck and shoulder muscles.
Is your head straight, tilted forward or back?
If your head is tilted forward or back you're doing the same as above. The more you think you are relaxing the more load your noggin puts on your neck and shoulders.
Do you hang your arms or rest an elbow on your knee or thigh?
Hanging your arms is neutral and shouldn't aggrevate anything but resting your arm on your leg makes you lean forward and doing it while keeping a hand on the throttle makes you twist.
Simply exercises and stretches involving your shoulders and neck will help a lot and working your core is also helpful. I'm .5" taller than you and have the same weight/inseam and I had the same problems until I put XC bars on my XR. My problem is/was increased by the facts that my neck is partially fused (3 discs) plus as of my lumbar discs are riddles with buldges and tears. For me the only tolerable position is sitting straight up because bending forward aggrevates my lumbar discs and makes me tilt my head back slightly (my neck only has about 25% movement remaining).
Retired USAF, IYAAYAS!
2008 V-Star 1100 Custom Bobber project
2011 Crimson XR with Forged Highway Bars / Hard Bags / Passenger Backrest, Ness parts including Smooth Layback Plate Frame/Mount / Rad 3 Mirror / deep cut grips / drivers floorboards /shifter peg / brake peg / passenger pegs. Witchdoctor black rack, Lloydz filter / timing wheel / VFC III, 7Jurock windshield, Drag Specialties seat.