Shoulder Pain During Run

I had a similar problem, pain in my right shoulder when running.

For me it was all posture.  If I look down at the road or slouch my shoulders, my traps/shoulders hurt really bad (at one point, it got so bad that was my limiting factor in how far I could run!!!).

Once I started looking ahead more (keeping my neck in line with my spine) and pulling my shoulders down/back, it all loosened up.  It felt really weird at first (since I had very poor posture before I began working out), but I got used to it, and my pain went away!

You can look up Chi Running - it helps with the posture of running and whatnot.

couple of posture questions:

do your arms cross the midline on your stride? that will put tension on your core as well as tire out your arms. They should not be moving laterally at all when you stride.

how tense are your arms? are they bent at <90 degrees at the elbow and loose on the stride, or are they in a death grip and overextended at the elbow?

Are you broken at the waist, or is there a straight line from the top of your head to your coccyx? Breaking at the waist will throw everything else off, and your arms might try to compensate to keep you in balance.

Do you do any upper body strength work? You might benefit from hitting the weights now and then, not necessarily at 'roid 'rage intensity.

How much tension are you carrying in your platysma (muscle in the front of the neck right under the skin) and trapezius? If someone saw your face, would they think you're taking a big ole poo? Being really tight in the platysma/traps will also put stress on your arms.

Are your hips balanced when you hit the ground, or are you favoring one side more than the other? again, weird postures/movements will put stress everywhere b/c your body will try to compensate for unnatural movements.

Are you landing on the ball of your feet, heel slightly kissing the ground only for a split second, or are you landing on your heels with the leg fully extended? The latter will lead to all kinds of injuries all over your body.