@North Coast Miller
Here are my thoughts/opinion on your predicament.
Mil Spec packs (I have used them all since the Alice Pack) are worse than what is commercially available, so do not go that route.
Apart from toughening up and embracing the suck, I believe there is a solution. I have had the same problem you are experiencing, I used to have bruising for days on long treks. This is how I solved it, and it works even when I carry loads in excess of 250lbs, except at that point my glutes feel like they are going to rip in half after about a mile. Anyway, I digress.
I am pretty certain that the load is not being properly distributed to your hips. Please refer to the awesome graphics I whipped up in about 10 seconds. Forgive the rudeness of the diagram. Most commercially available and mil spec packs, do not have IMO, proper load lifters. Look at the first bubble I drew, most load lifters do not actually lift the load sufficiently from the shoulder, thereby allowing the pelvic region to be the center of load bearing and power it was meant to be. Now the vector is not exact, as I stated, this was done in 10 sec, but that is generally the force vector that is being applied when the load lifters are too low. In layman's terms, it puts a lot of load on the shoulders, which hurts, sometimes a lot; and no matter how much I cinch my hip belt that vector is not changing. Then look at bubble two, if the lifter is elevated on the pack frame a couple inches the force vector changes (very roughly) to what is shown. This takes the pressure off the shoulders and puts it on the hips where it should be, and your shoulders will never be happier. When I made this change, it revolutionized my happiness when rucking. Just my two cents, hope it helps.