stuck in park

I have a non-running '92 leBaron convertible (v-6, 4-spd auto) in my barn. When I tried to move it, I found that it wouldn't come out of park. I disconnected the shift cable at the transmission and found that the shaft for the shifter won't turn. There isn't enough room to get a pipe wrench on it or to tap it with a hammer to try to shake it loose, and it doesn't seem like a good idea to try to heat it with a torch.

I've been assuming that what's holding it is corrosion where the shaft goes into the casting, and all I can do is squirt penetrating oil on it every time I go by.

Any other suggestions? Is it possible that the problem isn't corrosion at all, and there's something internal that's keeping it from moving?