It could be a few different things.
Have you been mudding or stream crossing lately? That could block the radiator and A/C condenser fins compromising cooling efficiency.
The thermostat may be sticking, causing the engine to run too hot, which can make the fan run excessively.
The coolant temp sensor may be giving the computer a higher than actual temp reading, making it kick the fan on when it's not needed.
So, look at the radiator/A/C condenser fins for any sign of blockage.
If that's OK, buy or borrow an infrared temp gun and point it at the base of the thermostat housing while the engine is idling and warmed up. The fan should come on when the temp is about 215°F, and turn off when it drops to about 195°F.
2008 Unlimited Rubicon Automatic