The maintenance schedules in the manual state to "inspect" the front and rear axle fluid every 24,000 miles and to replace if vehicle is used for police, fleet, trailer-towing, etc. So by following the manual's recommendations you should be inspecting the fluid every 24,000 miles and replacing if necessary.
On WK2Jeeps (Jeep Grand Cherokee WK2 - Maintenance information and schedules
) it shows that for diff fluids should be changed every 36,000 miles for vehicles operated under "Service Interval A" and 18,000 miles for vehicles operated under "Service Interval B".
You should at least inspect your fluid at 24,000 miles and replace it if it needs to be.