Poltax -
I don't have many posts but have been following on Rockslide for quite some time now. I am an Orthopedic surgery resident and your post peaked my interest for a clinical standpoint (sorry to be a geek on here, I mostly enjoy reading all the good info on hunting/backpacking that others provide but I may actually add something unique to this forum). I think I can answer you dilemma if you can answer a few easy questions: Do all your fingers go numb or just specific fingers? Is it one side of your forearm or the entire forearm? Does the hand/forearm also get cold? Do you ever have symptoms while not wearing the pack (i.e. driving a truck or at night when sleeping?)
What your describing could be a result of: 1. a pinched nerve in your neck (cervical spine), 2. pinched nerve in your arm (cubital tunnel, carpal tunnel, or a mix), or 3. potentially a blood flow issue as mentioned above. If only certain fingers/one side of you forearm go numb I would favor a diagnosis of either 1 or 2 (honestly this is wear an exam would help delicate between the two). However, if you entire hand/forearm go numb I would think this is likely a blood flow issue from the shoulder straps on your pack compressing an artery traveling to your arm (brachial artery likely). It is certainly worth adjusting the pack to see if it helps before pursuing a different diagnosis at this point, but keep in mind this could one of the other diagnoses as listed above (especially if this becomes a persistent problem).
This may be of some help. Good luck with everything. Happy to answer further/future questions.