One problem related to differential unit has been reported for the 1998 Nissan Frontier. The most recently reported issues are listed below.
While driving 60 mph differential locked up. This caused the vehicle to veer off into a field and hit a fence. Consumer had the vehicle towed to the dealer for analysis, and mechanic noticed that the differential was worn out and needed to be replaced. The bearing(s) were burnt and there was a fluid like sludge as a result of the heat.