Hit it with PB Blaster and it soak for awhile. 
Loosen the nut until its almost off then try to turn the bolt. 
If it doesn't want to turn then jack up the frame near the back while trying to turn the bolt.
Usually that will take tension off the bolt then it will turn. 

Once you get the bolt turning, spray some more PB in there.
Put a socket back on the nut with a short extension and lightly tap it while twisting the bolt back and forth.
If you have reduced the tension enough,  the bolt will slide with minimal effort.