Is it bothering you, physically? Many guitarists use different techniques on picking, some use the most limited motion, others use a lot more motion. The benefits to using the least amount of motion would be stamina. If you're not moving all over the place, you'll be able to do it a lot longer.

I don't understand really how your arm and wrist and hand can be relaxed but your shoulder is tensed? Maybe you're trying to play it too fast? Hence you get tensed trying to go faster than you're able. Comfort is an important part of playing and will help you in the long run.

Explain the situation a bit more. Does your shoulder tense up during everything you play? What about when you're strumming chords? Alternate picking?