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Hi Matt,
I always try to wait until the artist comments before "piling" on.
Tension can be caused by several things. The trigger for the tension does not have to be directly related to the neck muscles.
On of the main reasons for tension is an intonation aid. Many instruments are built in a way that we need to "pivot" or tense something up to squeeze the high notes a bit higher. Another possibility is tensing up to create higher pressure which makes exteme register possible. Another reason is the tongue getting in the way..................
When approached by a player with tension issues, we go back to my basic breathing exercises. Inhale and then immediately exhale without holding the air in. Try to think of breathing as a big circle - no defined point from inhale to exhale or exhale to inhale- just a smooth transition. Then we replace exhale with play. Long tones WITHOUT the tongue (even the extreme high range should be playable this way). Slurs without the tongue.
The idea is to break the playing up into basic functions in a specific order. We do not need an attack of the tongue to produce tones, so we leave that out when looking for problems. Once we are sure that our breathing is "smooth" and controlled, we cautiously add the tongue, paying attention if and where tension creeps in.
Without seeing AND hearing you play, it is hard to help you pinpoint the specifics. If you troubleshoot through the basics, you should be able to find the problem reasonably quickly.
One other thing: keep a glass of non-carbonated water in the practice room. If you notice tension, take a sip, swallow and try to play again. That helps me relax sometimes.