First of all, congrats on finally getting started on losing the "baby weight", Mike. (Usually the mother does this, but I can see how some fathers can pack on a few extra pounds during and after a pregnancy.)
I have been getting a similar irritation on my lower back, but that is because I keep getting spruce needles down the back of my neck while bushwhacking, and they get ground into my skin near my waistband and under the base of my pack. Not much I can do about that but pray for cooler weather so I can keep my collar "popped".
I would recommend two things for your "back issues":
1) The bodyglide can't hurt. If you are chafing in a certain area, applying a lubricant can never hurt. Think about it... if it works on your thighs, why wouldn't it work on your back? You'll need to find a good friend to get some of those hard-to-reach areas, though. (On your back, I mean - get your mind out of the gutter!
)
2) Cotton won't kill you. The Coolmax is great for wicking sweaty moisture away and evaporating it, but it leaves behind a salty residue. I've always felt that rubbing salty synthetic fabrics against my skin causes the worst chafing. I have found that a soft, comfortable cotton t-shirt feels better against my skin sometimes because the sweaty area stays moist and the salt-to-moisture ratio doesn't increase as much as it would with a poly-pro shirt. Your safest bet would be to bring a dry poly-pro shirt in your pack in case you get stuck in some inclement weather, but wear a cotton t-shirt while you are hiking. You'll still sweat, but the softer material shouldn't cause as much irritation. Throw the Coolmax shirt on for the second half, or bring a spare dry cotton t-shirt to change into if your body temperature is stable and it is working for you. Some people even recommend wearing a fleece base-layer.
It goes without saying that making sure your pack is well-adjusted and all is a good idea. If that pack has worked for you this long, you shouldn't need a new one now.
Anyway, watch out if you're gonna tell everyone you're losing weight - we might start calling you VFTT's Biggest Loser!