Soak the splines with penetrene today, tomorrow and the next day. Don't try again until it's had a nice long bath in it.
Do as Duncan says ref the nut and LARGE hammer. You want mass, not speed. ie, air hammer is bad. Use a drift to also protect the nut.
Clean the threads on the shaft with a small file, not sandpaper.
If you have lost threads out of the nut, replace the nut. In which case, do not fear damaging the nut with the hammer.
Use plenty of anti-sieze on the splines when you put it back together. Running them dry is 90% of the cause of the situation this car is in.