QUOTE
After long orchestra rehearsals I find my neck and shoulders are really sore
For a real test of stamina, try to get yourself into a rehearsal of Mahler 6. most violinists and violists (and not just amateurs or students) complain about shoulder and neck problems after long rehearsals.
One reason this happens is because the arm and hand position partially restricts the blood flow to the muscles.
A few Alexander Technique sessions will help improve things and release the tension, but the only other advice I can give is to take the violin away from your shoulder whenever there are sufficient bars rest. It will just offer a few moments of relaxation and restore the circulation to all the muscles.