How do you fix a broken nutt in the uni body connected to the rear springs?

I removed bolt from rear shackle. Front bolt connected to frame was rusted to in the bushing. When trying to unscrew bolt, it would pull of the framing. Loosened the bushings but at the same time had broken the nut in the uni body. Now the bolt only rotates. Any suggestions on how to remove and replace with out cutting into the frame.