Ugh, I'm in a similar spot with a very unchallenging, unfulfilling job. My dh has suggested that I do exactly what you've done: remove my Masters degree from my resume at a minimum and maybe not list that I got my BA at Berkeley.
I've generally found that it is not to my benefit to speak to the interviewers using more than low-average vocabulary or sentence structure either. I do generally modify my speech depending on who I am speaking with b/c I think it is rude and arrogant to talk over someone's head but I've not modified my speech patterns when interviewing with people who seem to be quite intelligent themselves. That resulted in one interviewer suggesting that I go back for a PhD and another stating in the interview that she feared I'd be unchallenged in the position. I didn't get either of those jobs.
Have you already accepted a position? If so, I guess that continuing to be the person you were in the interview is your best bet in terms of fitting into the office environment. I'd also find ways to keep yourself mentally alive outside of work. I've tried by doing some stuff on the side with gifted kids, volunteering at the humane society, writing occassional articles, and getting together with a group of people from my local Mensa group with whom I've clicked periodically.
