The rear of my 2005 F-150 FX4 seems to be sitting at an angle: left side lower than the right. It's obvious and not an illusion: everyone asks what's wrong with the truck. I measured from the top of the tire to the wheel well and the difference is about 2". I took the shocks off to eliminate them from the equation. The spring mounts look fine and the same on both sides, yet the left spring is "sprung" less (makes sense): measuring the distance between the lowest leaf of the springs at the front and the next leaf shows about 1/8"-1/4" difference.

Has one spring or the other failed? What else could cause this?