I can't recall hearing him called that. Then again, I live pretty far north in a rather liberal city. So that might have something to do with it.
Limbaugh: I Hope Obama Fails - The Rush Limbaugh Show
After Five and a Half Years, "I Hope He Fails" Remains the Most Memorable Expression of Opposition to Obama - The Rush Limbaugh Show
Rush Limbaugh's Shocking Words for President Obama | Fox News
I didn't read any of these because as I've said, I saw the original video. If you want other sources, just google "Rush Limbaugh wants Obama to fail"
No, he wasn't called a "nigger" all that much that I can recall. People just opted for "Kenyan" instead.
I've heard it on tv talk shows, more than once, and was frankly shocked. I've also heard it once while out shopping, from someone I didn't know. I suspect that if it is bandied about frequently, it would be a groups of like-minded people since such a characterization of POTUS would be incredibly offensive to most people, regardless of their political affiliation.