I have a similar issue, but mine sits higher only on the driver rear. Maybe 3/4" difference between passenger rear. I just raised the passenger side a tiny bit to compensate. Driver side is dropped as low as it goes w/ one lockring removed in rear. Passenger rear still has both lockrings. I'm probably going to raise it some though, getting tired of having the rear tires actually rub on the wheel well.
If you're not compressing the passenger side anymore, it sounds like a shock issue. If you don't adjust the shocks to compensate for the drop in the rear, you will bottom out the shock. On the front, it shouldn't matter since you're moving the shaft when you lower it. That's my only guess.
I think it would be very hard for the rear spring to move and not cause other issues. If you don't have enough preload, it could move but even with preload adjusted wrong, it would be very hard for the spring to change the ride height. The way it is clocked in there is pretty sturdy and even if it moved slightly, it wouldn't change ride height. The entire spring mount would have to move where the indentation is and that's like an inch. If the rear spring unloaded enough to do that; I'd be very concerned.