If your position is good, then indeed being checked for a leg length discrepancy is in order. I have a large one, and I did get upper back (not really neck, but upper back pain) until I got good PT for SI joint damage AND a custom crank set that deals with my LLD (but mine is very large, 3 cm, smaller ones can be corrected by easier methods). My PT also put me on a program of stretching and back strengthening exercises, which cannot hurt. It is hard to track these things down, since there can be many causes, so the best advice I can offer is putting yourself in knowledgable hands in terms of bike fitters, physical therapists, etc. I still think that trying wider bars may be an easy next step. Its not hard to change them out yourself, so it doesn't have to be expensive. You just have to learn how to move the levers and re-wrap the bars. I found Lennard Zinn's road bike maintenance book an easy way to get started doing my own repairs, which is essential if a lot of tweaking is in order.