The air ride suspension compressor on a 1998 Ford Expedition is typically located
underneath the vehicle, near the rear passenger side. It's often tucked up fairly high and might be somewhat difficult to see without getting underneath the SUV. It's usually attached to the frame rail.
However, the exact location can vary slightly depending on specific trim levels and options. To be certain, consult your owner's manual or a repair manual specific to your 1998 Expedition. A visual inspection from underneath the vehicle is also recommended.