Highly doubtful on the stated fuel economy #s.......I have a 2012 ML350 BlueTec...similar setup with Grand...7 speed vs 8 speed and I am averaging 24mpg right now. If the Grand gets the 5 speed there is no way it will get 33mpg...My 2008 Grand Cherokee CRD with the 5 speed is above 2500rpms when going faster than 65mph.
Check out my fuelly page....
http://www.fuelly.com/driver/scootr29/ml350
I used to have a 2010 ML350 BlueTec and I never saw above 28 mpg.
The E350 BlueTec struggles to get above 30 mpg and it is a lot more aero than the Grand. Aero is the biggest disadvantage for and mid size diesel SUV...if you bought a 2wd Grand or ML350 that will bump you up slightly.
Now all that being said...if you bought a 2013 Grand or ML350 and drove it strictly at 55mph and no city driving you may break the 30 mpg mark.