Anything's possible. But he said he first heard, then found a crack. I assume it's the exhaust manifold.
The exhaust pulses, especially that close to the head. Most of the time it's pressure in the manifold, but between pressure pulses it actually goes negative pressure. That's the principle used by Pulse Air Systems, even headers work on the pulsing principle.
We don't usually think about that negative pressure, but it's there. Even stock manifolds use that principle to scavenge the exhaust from cylinders - just like a header.
Sometimes we can see that negative pressure at the tip of the exhaust pipe too. Hold a piece of paper against the pipe's tip, you can see it flap, especially at a slow idle. But sometimes you can't see it, because the long pipe, cat, and muffler act as a big damper.
When it's during the negative pressure, it's pulling air in. The Oxygen in the "false air" gets on the O2 sensor(s), giving a wrong signal to the PCM, causing the engine to run funny. Not only mixture but timing can be affected by that extra O2 that wasn't from the engine.
Yup, no carbs on TJ's, but he's probably thinking of the throttle body.
Tip on finding an exhaust leak up front - dark night outside, strong pencil beam flashlight like a mini-mag lite, and carb cleaner.
Turn out the lights, pour a little carb cleaner down the throat - it'll smoke. Shine that pencil beam around the manifold, head pipe, O2 sensor etc - if it's leaking you'll see the smoke from it and the source.
Be careful not to breathe the fumes - they can be deadly.