My Suburban barely fit on a flatbed, both length and width, doubtful a dually would fit. As Bryan said, have them either shift the transfer case to neutral or lift it from the back. To shift to neutral turn the ignition on (don't start the engine but assume that's not an issue if it's getting towed LOL), step on the brake to put the shifter in neutral, and hold the 2HI and 4LO buttons for 10 seconds until the red neutral light comes on.
Reminder, Park will no longer hold it from rolling if the t-case is in neutral.