Quote 'safe' or not... the centre diff is an open style, and aided with ABS traction control. if your centre diff was fully locked at all times, you would need identical OD tire sizes. now, I'm not specific with the xdrive, but the centre diff in the 4.6is is pretty simple so that's all i can speak too.

__________________
2003 4.6, esotril blue, over 215,000KM. Every previous owner failed to keep up the vehicle maintenance. Restoration project.