This nut is "jammed" meaning, that alone, the nuts are just that, spinning freely up the bolt. Once they are brought together and tightened using opposite force, they become unmoveable and thus your shifter will not spin.
To unjam the nut, just do the oppisite. Sit in the drivers seat, take 2 wrenches, put one on the bottom nut at 4:00 and another one on the top nut at 8:00. With ONE HAND, squeeze them together. This should break the jam and you can then remove the nuts. At first you think you are tightening them, just keep with it. Watch your knuckels, Good luck.