I dont have an answer but I do know how you feel.
Alot of the time I feel like my tongue is tied in one big knot and my words come out backwards and I have a hell of a time trying to explain(find the words)I am trying to say.

IMHO - After a day of research, I beleive that this is biologically similar to fainting. Norepinephrine (aka noradrenaline) is released into the system. THis is part of the 'fight-or-flight' response to panic. In your body, this is meant to stiffen muscles in preparation to spring into action.
Fear and numbing may stifle your ability to act once your body is prepped for battle. This may inhibit speech.

http://www.surgeongeneral.gov/library/mentalhealth/chapter4/sec2_1.html

I've done a little research on this subject myself because I have been through the same symptom. The words are there in your head, almost screeming to get out, but no sound will come out of your mouth. Not only frustrating, but scary too.

According to Bessel Van der Kolk who is the leading researcher in PTSD in the country, when trauma reactions such as anxiety or triggers, flashbacks and dissociation occur, the part of the brain that stores the traumatic memories overrides the part of the brain that relates to speach. It is a natural reaction to extreem events in your past. It is nothing that is your fault. However, if you can give voice to the people who try to hug you when having an anxiety attack, I thing that you would feel better. PTSD is always about safety and when you are in an axious state, safety is what we need. The hugs would violate that feeling of a need for safety and make you want to withdraw. I don't know if that is exactly your experience, but when I came accross the works of Van der Kolk, I felt alot better because finally there was an answer to a lot of my questions. Hope this helps.

I have the exact same issue. When the feelings are scary, I feel like my speech centre in my brain just shuts down. Sometimes I don't even have words in my head- just this non-verbal terror. And I totally get the " I need to be alone but I don't want to be alone. It feels like I'm being pulled in 2 directions, by me. I found one thing that helps in terms of the reactions of others. Test the waters out with people, don't trust them with the whole enchilada of your feelings. It's devastating when they reject you or don't get it. It helps to weed out who the empathetic listeners are. And the other thing I had a hard time getting is that someone might be very sympathetic one day, and not the next. I think sometimes it's because they are not always in the same place themselves. They are like everyone else- good days and bad days. So try not to totally withdraw if sometimes they don't give the response you're looking for. Actually, it's quite ironic that I wrote that, because that's one of my biggest issues. If someone hurts me- even unintentionally, I completely withdraw. Oh well, just another thing to work on. That's ok. Hope this helped.