Injury to the long thoracic nerve although not common may occur from repetitive microtrauma with heavy effort above shoulder height, pressure on the nerve from packpacking and vigorous upper limb activities. If pain,weakness and winging of the scapula ensue, this is not a good sign. Recovery time mya take up to 2 years. So if i were you, elimination of the precipitating factor is a must which is your hiking and backpacking. In this case, you should focus more on your health over your passion as this will take toil later on in your life if you don't stop temporarily.

By the way, here's a link that would somehow help you understand why this happens among backpackers:

http://www.ncbi.nlm.nih.gov/pmc/arti...00238-0091.pdf

CHEERS!!
Charlize29