Lets turn the joke around and see if its still funny. "I am on this thread to talk about Ann Coulter but you have to go into rehab if you use the word "tranny" so I will have to pass."
See its not funny because unlike "faggot" you don't have to go into rehab if you use the word "tranny".
She was just making a point about political correctness and the fact that the guy from Grey's Anatomy actually went in to rehab, or shall we say reeducation camp, because he called a gay guy a faggot on the set. And how many of the people who heard Coulter knew that background? Or instead just thought she was screaming for attention, again?
The more you have to explain a joke after the telling the less funny it is.