I went through this with my neighbors Craftsman Lawn Tractor. It was not easy. Tried all the methods here. Heating, cooling, penetrating oil, gear puller, hammer & chisel, etc. I did not want to pound too hard on the rim from behind as I worried about the transmission.
I went to a friends service station to ask him for advice, and told my friend I would be back soon. He decided to beat on the back of the rim with a chisel and big hammer while I was gone. When I got back my I found he had knocked something loose in the transmission, as the axle now moved. This was a hydro-static transmission.
So anyway, I had to pull the whole transmission out and take it all apart. On the good side I was finally able to get the rim off now that I had the axle out, and fortunately the only damage was to an internal 'C' clip. New rim was needed, that old one did not want to move after 20 years. Mower's transmission is still working well 3 years later.