If you are in the middle of nowhere with no heat, no time to wait for a penetrant (WD-40, PB, etc) you might use the technique I used:
First, make sure that the threads on the lunette are clean. File out any burrs that might damage the nut because the nut is impossible to replace.
1. Find something, like a 3# hammer head, that will just fit between the end of the lunette shaft and the frame.
2. Screw the nut off so that it will start applying pressure to push the ring out.
3. Seriously whack the frame above the lunette using something like a 7# hammer.
4. Tighten the nut against the block.
5. Repeat steps 3 and 4 until the lunette pops; it is tapered so when it pops it will just slide out.
This has worked for me on several seriously rusted lunettes. When you get it off, clean it and put axle grease on it so that you don’t have to fight it again.
Arlyn
PS If you do damage the nut, threads on the lunette are 1 1/8 by 16 TPI. The large lug nut on a 16 wheeler is the same thread pitch and diameter. Make sure they don’t give you a left handed one and buy 2, they are thin.