Changing axle fluid is easy and only costs about $20 compared to $50-$90 at the shop. It also is a great time to inspect your jeep to avoid a possible disaster down the road.
Cars/Trucks/Jeeps should have their axle fluid changed much more frequently if they are used in off-road or towing situations. If you have any thought you may have gotten water into your housing change your fluid immediately.
This writeup is written for a 98 Jeep TJ however it should apply to any vehicle. (Check your own manual for correct fluid types)