Where exactly are you driving on the sand?
There's nothing that is more difficult to drive on than loose, dry sand. Just watch some of the Dakkar rally and you'll see. One second you're going along fine and then you lose some momentum or give it a bit too much pedal, and you're suddenly buried up to the axles.
One thing that you'll notice if you head out to any dunes, the vehicles that are out playing there are light, have wide tires (that are aired down) and a decent amount of horsepower. Now look at the Expedition... it is none of those things, especially in stock form.