Have someone turn the steering wheel back and forth.. not a lot, just the area that has all the play in it... I could turn mine back and forth about 4 inches and not have anything happening under the Jeep.
Hold your hand on the pitman arm. It is far better to put hands on it, than just visually look for movement while the steering wheel is being played back and forth. You can feel the slightest movement before you see it. If there is excessive play at the wheel with no output from the steering box, it might need to be replaced. As 7D8CJ stated above, the coupling from the steering shaft to the box could be worn out as well.
After debating about replacing my steering box, I swapped it for a reman from a J20 pickup. WOW what a difference. It felt like driving a sports car in comparison to how it used to handle. Its nice to have a jeep that responds to steering imput.!!
good luck