The '91 notch is sitting about an inch and a half higher in the back on the right side then on the left. Replaced both rear shocks could this be a bad spring on one side?
The Rear Springs should have the Pigtails to the Rear and pointing toward the Drivers side. Your Front Springs can actually mess up the rear height too, make sure they have the Pigtails in between the 2 holes in the pocket in the A Arms.
Almost all mustangs I see sit higher on the pass rear than the driver rear. Even after new shocks, coil-overs and lower control arms my GT is an inch lower on the driver's side. I too would love to know how to cure this.