Ground glass stoppers that are stuck can be difficult. You may, after soaking with WD-40, put the body of the jar up to the lip in a warm bath. Gradually warm the bath to hot (I say, gradually). The increased air pressure inside the vessel may allow the stopper to pop free.
If that sounds like a good idea, you could heat the interior air even more by submerging the jar in a sauce pan of water. Keep the jar off the bottom of the pan with wire 'legs.' You may have to attach some lead wheel weights to the legs to prevent the jar from bobbing at the surface of the water. Slowly bring the water to a boil. As long as the glass is heated uniformly, it should be fine. Handle with oven mits, and don't let the jar cool too fast.
Heating and cooling glass is done with some risk. Glass will tolerate substantial heat, but you can't apply the heat suddenly or differentially without risking fractures.