Rear wheel removal 6x4

I have a flat on the right rearmost tire of my 17 yr-old 6x4. When I tried to remove the wheel and after removing the lug nuts, I found that the wheel impossible to remove. Tugging at it, whacking it with a rubber mallet--nothing would make it budge.

My question: Is there something other than the lug nuts holding the wheel to the hub? Could it just be rusted tight since it's never been removed?

Any insights most appreciated.

Aloha.