Conversely, it may also be as simple as lowering your bars. Another BF member once posted about how his upper back pains were relieved by dropping his bars so that more of his weight was carried by his torso instead of his arms. It sounds counterintuitive, but it's what worked for him.

What had been happening was that the bars were high enough that he didn't have to use his core muscles at all to stay up -- it was all shoulders & arms, so his neck and upper back would start to sag (think of shrugging your shoulders backwards).

Try the same effect with your posture, if you haven't already. Relax your arms, ride with a little bend in the elbows, and roll your shoulders back, down, then a bit forwards. Don't let your body hang between your arms.