My car sits about a 1/2" high on the driver side rear. I never really noticed until I was ready to do my suspension so I measured from the center of the wheels to the fender lips to get some reference numbers and found out that the driver side is 1/2" high. At the time I never really thought much of it other than maybe I had a soft spring and figured that my new springs, dampers, and adjustable panhard bard would solve this problem. It didn't. The driver side rear is still 1/2" high.

Any ideas?

 

I think all the mustangs are like that. I know mine is the same as yours. I measured all four corners, before and after lowering. Don't sweat it, not a big deal, for me anyway.

 

Make sure the pig tails of both rear springs are facing the same direction. I mean, it shouldn't be a 1/2" difference, but might even things a bit.