One problem related to axle assembly has been reported for the 1998 Ford Expedition. The most recently reported issues are listed below.
I have had the drive train from the transmission back replaced and still have a vibration thruoughout the vehicle. Along with this it is difficult to stay in your lane while driving. The fix from Ford was to hang a twenty or so pound wieght off the differential. Through the TSB on the veichle print out I was also informed to watch out for the tires spinning on the wheel form 45 - 80 mph (Ford TSB - special service message 11276 under vibrations concerns).