Semi-resurrection here, but I was going through posts and this one caught my eye. If you feel like you're sitting back and you are counteracting that by bending way forward at the waist -- well, yeah, that's gonna put strain on your lower back. If it hurts, don't do that.
The solution is that rather than moving your shoulders/head forward you need to move your hips forward by flexing more at the ankle and less at the knees. Then you won't have to fold over at the waist to keep your fore-aft balance in the right place. That's what the other posters meant by "opening your hips" -- not being bent over at the waist.
If you're not able to flex enough at the ankle to stay forward, something is wrong with your boot setup. It's possible that your boots are too upright and this isn't letting you flex far enough. (Even very soft boots will only flex so far from neutral.) If that's the case, usually the forward lean angle of the boots can be adjusted. Another less likely possibility is that your boots are leaned too far forward, which then forces you to bend your knees to avoid falling forward... and then to actually get 'forward' on your skis you have to overflex at the waist. Normally you only see that when someone is in crazy expert/race boots when they shouldn't be, though.
Too-stiff boots can also cause this, but from the description it sounds like these should be fairly 'soft'. However, if they don't fit well, even 'soft' boots can be difficult to flex. You said you "really tighten them up as tight as they can go" and have a low-volume foot/leg, so that could be going on. Normally good-fitting ski boots should be fairly snug even with the bindings unbuckled. If you have to crank them down hard to get them to stay put your boots are almost certainly too big. (If you pull out the liners and just check the 'shell fit' on your feet, is there much room? If there is much more than half an inch of empty space at the back when your toes just touch the front, the shell is too large.)
Pics of you in your boots on a flat surface from the front and side both 'neutral' and flexed forward might be helpful.