It's not adjustable and if it's out of specs, it's most likely that one or both rear control arms are bent. I've seen that a few times now and I don't know how they get bent (accident, too many bad potholes, etc?) but when they're bent, you can usually see that one or both are not perfectly straight when you look up at them from below the car. If you compare it to a new part, it's then obvious.
Both need to be removed to replace just one and they're about $400 each (x2) plus about 8 hours labor including alignment.