I'd ask to see the crack...

There are only a couple things that will crack a manifold.
1. lot and lots of mechanical stress. You can pick up a motor by the manifold.
2. casting flaw
3. WAY over heated.

Same with an injector. Those things just don't see that kind of stress normally. I have seen cracked ones injectors, but 3?
Either your jeep has seen some serious abuse, or you mechanic likes to dish it out.