If it turns into a battle:
You can make it easier on yourself with a length of bar and a couple of large C-clamps.
Support the car on jackstands & jack up the diff, free it from the springs.
Clamp the bar to the spring, reduce the arch, and the spring will lengthen.
(You can also help the process by loosening the front spring mounts; the spring & shackles will move rearward a bit.)
Pop the rear shackles back if they're not there already, and then reassemble.
Torque to specs after the car's back on the ground.