Warhawk, I am the EXACT opposite as you are. I am great when it comes to speaking in front of large groups. I was in drama club in high school and have been in many different plays, and I love giving speeches. It doesn't really matter if I am prepared or not, I don't get nervous. Then again, if you told me to give you a speech right now on something I know nothing about, I wouldn't know what to say, but if I am even slightly knowledgeable on the subject, I will do fine.
When it comes to one on one, I freak out. That is why I have such a hard time getting anywhere with the opposite sex. I can only talk freely to them once I know I don't have a chance (like if I find out they have a boyfriend/fiance/husband). If they are single, I will have a hard time getting to know them because I am always self-conscious about what I am saying and looking like an idiot.
I guess I am not afraid of looking like an idiot in front of a large group because there is a pretty good chance that at least someone in the audience will find me funny. When it comes to one on one conversation, my jokes only have a 50-50 chance of being funny...and I make a lot of things not knowing if they will be funny or not.