The motion is side to side and happens when large vehicles pass on the interstate, going down hills and any time I get over 55 mph. The only reason I've been told about the air suspension is because it is an "air bag suspension" and works like a balloon. The air moves and therefore causes the sway.
The Expedition only has 69000 on it. One garage I went to about it said, "I get this concern about 10-15 times a year. Ford knows it is an issue and hasn't done anything to fix it. The options are to replace it with a different system for about $2,000 or trade it in."