I just read an article on SectionHiker.com that too long a torso length can produce the symptoms you describe. The author's suggested solution is an adjustable torso length backpack, and he lists some. This may or may not help, depending on the pack fit and how it's loaded. You might need another brand that fits better.
Those of us with really short torso lengths often have this problem.
Adjustable length backpacks

Just another item to keep in mind.

It might be a good idea to make an advance appointment with REI, explain your problem, and ask for their most experienced backpack fitter. In addition to your gear, if you can mock up the equivalent in weight and bulk of several days' food and fuel, and take at least a liter of water, that will help.

Also, I suggest you take a copy of Glenn's post above (#202919) and make sure the fitter goes through the list.

Good luck!