What's the worst that could happen?

So what's the worst that could happen if I let my cracked header go for a winter?

Yeah, yeah, I've done a search and read all about replacing it but here in Iowa Old-man Winter is swiftly coming and I'd like to be able to put off the job till spring.

Is there any risk?

How bad of a crack? If its a bad one, you could let exhaust fumes into the cabin which isnt a good thing obviously. If its just a little crack, id just wait till when its warmer.

It's not too bad. Just enough to be noticeable (you can definitely hear it). But like you I've imagined the worst; stuck in a snow bank in sub-zero temperatures and sucking in exhaust fumes.

Two risks come to mind. First is the fumes getting into the cabin. Second if the crack is bad enough and it is cold out cold air could be pulled into the engine when you shut it off cooling the valves to fast and causing damage. Never seen that happen, but it's one of those 'I been told' things.

Exactly. One of those "learn the hard way" type things I don't want to have to deal with. I'll be posting on this same forum with a entry that starts "So I let my cracked manifold go for too long..."

One thing you may want to look at, is which direction the hot gasses from the leak are blowing. There is a wiring harness down there. Getting it crispy can't be a good thing.
I stick my hand and arm up around the exhaust manifold, just to get an idea if what works and what doesn't on a cold motor with engine off.
Then start it up and stick my hand up there to feel where the exhaust gases go. You only have a very short window of opprotunity to check things out, before things get too hot to handle.
On a really cold morning, you can see the exhaust fumes/leaks pretty well. A good time to check for exhaust leaks in the whole system.
It's been my experience, a drafty rear hatch (seals or bad fit) will fill the insides of the XJ up with exhaust fumes pretty quick, a leaky exhaust manifold less so.

A cracked exhaust manifold can also ruin the upstream O2 sensor. You may be buying one of those next spring also. Just FYI.