I thought all foxes leaned from the factory.

lol! all foxes lean to the left. I have yet to see a fix for it. My 87gt did it, my sisters friends who owned mustangs all leaned to the left.. it's just a common thing.

I haven't read all these responses so forgive me if I'm being redundant but what I see is a focus on the rear of the car.  Problem is most of the lean on these cars has to do with the front suspension, not the rear, although it'll make itself more evident in the back of the car.  The reason is because there's so much more weight on each front spring than there is on each of the rears, and any drop in one of the front springs has an effect on the corresponding rear spring.  In fact it may be raising the opposite rear corner.   I'll let you know when I get around to fixing mine, which is going to involve a small cut to the right front spring to fix a lean to the left. 

I'm not saying that the springs are the exact cause of the lean but if you want to fix a lean to the left at the "rear" of the car, you either need to raise the left front or lower the right front.   

If you try to fix it in the rear all you'll do is jack the rear higher in the air and the lean will still exist.  Been there done that.  I tried to fix a lean on my car with a spacer on the low-side rear spring and all it did was raise the entire rear end a corresponding amount while leaving the lean in place.   And just recently I replaced the rear springs to bring the whole assembly down a bit, and the springs are exactly equal.  So are the fronts, and yet I noticed when all was said and done the lean still exists.  It may be a structural issue of some sort but if it's going to be fixed it has to be fixed in the front.

Either raise the low side front with a spacer or isolator or lower the high side front with a cut or what have you and when you level the front, you'll level the rear.