I'm a newb too....but I might be able to help you on this one.
Did you have the rear tires off of your truck? If so...take them back off and make sure there is a little pin in there behind the nut that your wheel pushes on to.
I just got an MT 2 a few weeks ago, and it has been a great truck...but the first time I had the rear wheels off, and then went out and ran, I only had front-wheel drive. I didn't realize the pins had fell out. They are very easy to miss if your new and didn't know they were in there to start with.
Chad