Are you gripping the bar with your thumb around it? If so, don't use the thumb; keep it in the same direction as the rest of the fingers.
Also, how close are your hands to your shoulders? Have you tried keeping your hands closer to the plates (not an ideal position, but could help in release pressure.)
The most likely cause is that you have poor internal shoulder rotator inflexibility and upper thoraccic mobility.
Insufficient upper thoraccic will stop your shoulder blades and your upper spine from moving properly as you bring your hands back and overhead/behind the neck. Insufficient internal rotator inflexibility will stop your forearm from rotating backwards to put them into position. So basically you are "americana" yourself.
I would suggest that you immediately stretch your internal rotators, your external rotators and strengthen your external rotators by doing these in the given order:
http://www.exrx.net/Stretches/Subsca...roomstick.html
http://www.exrx.net/Stretches/Infras...SideLying.html
http://www.exrx.net/WeightExercises/...lRotation.html
Coincidentally, stronglifts just published one article that you may find useful:
http://stronglifts.com/squat-bar-position/
Check also the following:
http://stronglifts.com/shoulders-dislocations/
http://stronglifts.com/how-to-improv...acic-mobility/ (go to section #2)