Will the utility box's that are made for 2005-2010 still work with the new frame? The stealership told me the frame changes are in the front but the rear frame remains the same.Does anybody know?

I would call some of the utility body manufacturers, I bet GM has given them frames to work with, they will know.
As usual your dealer knows nothing about the new trucks.

The frame is DEFINITELY boxed at the rear end. You can see it in the videos on YouTube, and also in the pictures posted in redeagle313's garage, here:
http://www.dieselplace.com/forum/garage.php?do=view&g=30040
He has a new 2011