I think that writing and speaking (on an informal level) can go hand-in-hand. I know that my writing is not great. It was always my worst subject in school. I'm also not a great storyteller or joke teller as some people have had to suffer through my attempts. I've killed many a good joke.
I find I have the same issues with writing and talking.
1) My mind moves to fast for my hands or mouth.
2) I lose words. I will be in mid-sentence and either lose the word I wanted to say or forget what I'm saying. I thought working on increasing my vocabulary would help, but it just gives me more words to forget.
3) I'm generally too the point, but ramble to get there. So, there's the constant hurry up you're talking/typing too much. This is sometimes good, but often times confusing because I will leave out important parts of my point.
That being said, I'm awesome at writing kids books and speaking to the students that I teach. I'm definitely in the right area. I don't know what that says about my verbal/linguistic intelligence, but I think my weakness is a strength for my job. I'm just too scatterbrained for my own good I guess.
As for public speaking, with enough practice and rehearsing I can do it well. I got an A in speech when I was in college. I liked my cheat note cards.
I need those for life too I think.