Not quite, my MJ needed an engine at around 150k miles. It was knocking and I still drove it another 10k miles, roughly. It never blew up, I just found another to swap in. I would definitely do more diagnosing though, these engines are pretty easy to work on. That said, if this engine ever starts having problems, my truck is getting an LS. It's considerably more expensive, but its over 100 hp more, will result in the same or better fuel economy, and I've never seen one that needed more than a starter or exhaust manifolds, and I've been around a lot of them.
The 4.0 has an exaggerated reputation in my opinion. They're not powerful, they're not good on gas, and they can be a pain to keep from leaking. They're a great engine if you want to beat it and not have to take care of it, but don't care about leaks or power or fuel economy.
In short, my opinion is diagnose the problem first. If you can get away with fixing the engine, go that route. If you'll need to pull the engine apart and fully rebuild or replace it, drop an LS in it and never have to work on it again.