First of all, I feel your pain. Sharp, stabbing pain every step isn't within the realm of normal aches and pains, and you probably don't have to live with it.
My recc. is to see a podiatrist vs. the ortho. I have the opposite problem (very high arches). Custom orthodics, after a series of cortisone injections, solved my problem. I was reluctant to go that way, but after talking to a number of distance runners, I took the plunge.
As backpackers, we put a ton more stress on our weight bearing joints than the average person. Even if you're fit, adding a pack instantly makes you 20+ lbs "over-weight." Add to that the fact that you're putting significantly more mileage on those joints than the average desk jockey and you have an irregular foot strike, and it's easy to see where something's going to give.
Anyway, my advice is to see what a specialist says.