1) Hit it with PB Blaster and it soak for awhile. 
2) Take Nut off
3) Jack up the frame under the rear shock bridge until you reach a point there is no or little tension on the bolt (there is a point)
4) try to turn the bolt.  If to does not turn try turning the bolt while someone moves the jack up and down to find the sweet spot. If you take tension off the bolt then it will turn. 
5) If it won't move spray some more PB in there.
6) At this point I usually use a long 3/8 extension, put it on the end of the bolt (not the threads) and use a dead blow to start tapping it out.  Make sure you have the bike at the sweet spot on the height so when the bolt comes out the swinger & motor are not under tension and pop apart.

If you have reduced the tension enough with the jack,  the bolt will slide with minimal effort.


If it's as bad as you say it'll be new bearings and hopefully the sleeves are in tact and not grooved up from being stuck and rotating on the sleeves and not the bearings.

It can be a job the first time but do it right and keep it greased and it'll be much easier next time.