It sounds like when you protract your shoulder your getting neurological problems, could be you have increased neural tension. Go visit a physio and get them to asses your upper limb tension tests. The neural component cause cause your muscles to not contract correctly. Could be that they are weak.
a violent clunking sensation - depends where its coming from, if I'd GH joint then best to strengthen the Wikipedia reference-linkrotator cuff - but it depends on the area of instability as to which area to work the most. If it's AC or SC then again strengthening could help but it's not perhaps as effective.

I'd suggest getting a physical exam cause there are a few things going on there. And relatively hard to give u a diagnosis online without physical exam.

Pudding