I had a 99' Eddie Bauer Expedition. Drove the thing for quite a while and felt it would drift and sway alot. Like you, I assumed it was tire and shock set-up. But one day during an oil change I noticed there were no link pins connecting the front sway bar to the control arms!!! There was no evidence that these were ever installed at the factory. The dealer tried to tell me "they must have fallen out". The link pins are about 13" long, are installed and suspended from the top ... I would have had to lose both retaining nuts and then rolled the vehicle over for them to "fall out". I started looking at other Expeditions and noticed several the had the link pins missing. I had to buy the parts and install them myself. It was a totally different vehicle.
Does yours have the link pins installed?