* Transfer Case Issues: This is perhaps the biggest area of concern. Problems can include:
* Shift Motor Failure: The motor that shifts the transfer case between 2WD and 4WD can fail, leaving you stuck in one mode. This is often accompanied by a grinding noise.
* Internal Leaks: Seals and gaskets can wear out, leading to transfer case fluid leaks. This can severely damage the transfer case if not addressed.
* Bearing Wear: Worn bearings can cause noise, vibration, and eventually failure.
* 4WD Engagement Problems: The system might not engage properly, either due to issues with the transfer case or the front axle components (like the actuator). This can manifest as the 4WD lights flashing or not engaging when selected.
* Front Axle Problems: While less common than transfer case issues, problems with the front axle components (CV joints, axles, etc.) can affect 4WD performance. These often manifest as clicking or popping noises, especially when turning.
* Vacuum System (if equipped): Some 4WD systems in this era used vacuum to engage the front axle. Leaks in the vacuum lines can prevent proper 4WD engagement.
* Electrical Problems: Worn wiring harnesses, faulty switches, or other electrical components can disrupt 4WD operation.
In short: A 1998 Expedition's 4WD system isn't inherently bad, but age and mileage will inevitably take their toll. Thorough inspection before purchase, including a test drive in 4WD, is crucial. Regular maintenance, including checking and changing the transfer case fluid, is essential for longevity. If you're considering buying one, be prepared for potential repair costs related to these components. Listen for unusual noises, and ensure 4WD engages smoothly and reliably.
