I know quite a few people who develop some swelling in the hands (which will cause numbness) when backpacking--it's due to the hands hanging down all day. Using trekking poles (which keeps your hands higher up as well as exercising your arms) solves this difficulty for most. Some like to use loops for the hands that attach to the pack shoulder straps. This also keeps the hands higher, although it eliminates the extra exercise.

Since most of your backpack weight should be on your hips, your shoulder straps should be relatively loose--not so loose as to let the pack bounce around, but loose enough not to affect circulation.

If using trekking poles doesn't solve the problem, or if it occurs only in cold weather, then Reynaud's should be considered. However, my experience (my daughter has it) is that it is far more apt to occur in the feet.