Two problems related to differential unit have been reported for the 1998 Ford Explorer. The most recently reported issues are listed below.
Drive train failure, differential, within 12 months after having 60,000 mile differential fluid replaced with synthetic ,to Ford specs. Differential gears and clutch assembly disentigrated. Resulting in replacement of entire differential. Could have caused gears to lock-up while driving.
At about 73,000 miles the rear differential went out on my 1998 Ford Explorer.