If the radiator is full, the level in the bottle should go up as the engine heats up, and go down as the engine cools, staying between the max and min marks.
With a cold engine, remove the radiator cap and see if the radiator is full. If it's not full, top it up with 50/50 mix of OAT coolant. Also bring the level in the bottle to above the min mark.
Drive it and see if the level in the bottle behaves normally.
Look for signs of coolant leaks right after you shut the engine off, and again when it cools. Pay particular attention in the area of the water pump.
2008 Unlimited Rubicon Automatic