I'm currently laying here recovering from a microdiskectomy. I herniated my L5/S1. If you have a bad disc, you need to be really careful. It's a $25,000 surgery (I'll be $6k after insurance out of pocket this year, so much for hunting). After the surgery, you can forget about squatting, over head press, or deadlifting in the gym, extremely heavy packs, or any other heavy strenuous lifting... Unless you want to explode the disc and go back to the surgeon for spinal vertebrae fusion.
For sure try to adjust the pack some. And drop back the weight a bunch until there is no pain. There are other ways to gain endurance and strength without training yourself into pain like that. My suggestion with long lasting pain like that, you need to get an MRI. Your back isn't something to mess with. My doc visit to get an MRI referral was $35. With my insurance the MRI only cost me $250... If it's about money, that's only half the cost of just your pack. Much less what's in the pack, gun, tags, spotter, etc. If it's about being to manly to go to the doc, let me tell you, chicks don't dig scars. That stupid saying is bull$hit. To really be at your best during hunting season, and at life in general, you need to find the source of the pain, and figure out how to get rid of it. Go to the doc. Be honest about the pain level with him. Ask for an MRI referral. If it shows something out of whack, get a CD copy of it and see a spine specialist. That needs to happen before you're laying around cruising the forum recovering from a back surgery... like I am.