I've been dealing with the same issue myself. I tried all kinds of ab stuff, single leg work. hip exercises, etc. No squats for 12 weeks. I've been doing all kinds of lunges and front squats.
Today I said ENUFF! and just back squatted. I asked a big dude at the gym to watch me squat, and the first thing he said was "move your feet out." I moved my feet out, and BAM everything felt better. I could feel my glutes and hips working, and the guy watching said my squat was no longer crooked!
So I would totally agree with the advice to widen your stance. Like so wide your feet touch the bottom of the power rack. Try it, and see if it takes the stress off your back, and if you feel the glutes and hips more.
Also, maybe try to pull your elbows more forward and under the bar. This should help you stay tight and upright, so you don't lean forward in the hole.
Like knobby says, weights will probably go down at first. It may even be tough to get to parallel the first time, it was for me today.