Jeep Grand Cherokee WK2 - Maintenance information and schedules
Per the above link, should be replaced every 36,000 miles if you fall under service interval "A", or every 18,000 miles if you fall under service interval "B".
Service Interval "A" (Light)
Service Interval "A" is for vehicles that are not operated under any of the conditions listed under Schedule "B." Most vehicles are operated under the conditions listed for Schedule "B". Use the schedule that best describes your driving conditions. Where time and mileage are listed, follow the interval that occurs first.
Service Interval "B" (Heavy)
Follow this schedule if you usually operate your vehicle under one or more of the following conditions:
Day or night temperatures are below 0°C (32°F)
Stop and go driving
Excessive engine idling
Driving in dusty conditions
Short trips of less than 16.2 km (10 miles)
More than 50% of your driving is at sustained high speeds during hot weather, above 32°C (90°F)
Taxi, police, or delivery service (commercial service)
Off-road or desert driving
If equipped for and operated with E-85 (ethanol) fuel.