If it is on the manifold, I consider a weld to be just a temporary solution - it will probably crack again due to the pressure. If it is on the headpipe, you can probably get away with a plug weld. Your best bet on the manifold is to replace with headers, or turn the music up.
89 XJ 4.0 AW4
K&N 6x9 cone filter setup
Converted to TJ axles - 5-297 U-joints
Optima Red Top
If the crack is accessible enough, I would weld it. You can then plan for a better fix later after you decide how long this XJ is going to be part of your life. If she is a keeper, then upgrading to a permanent solution via an aftermarket header should give you the best chance of failure proofing. When I replaced my header, it was a pain in the rear to get everything retorqued down. Wouldn't want to do that again if at all avoidable.
Otherwise it would be a shame to invest a lot of time and money into a vehicle that might only be in your possession another year or two.
thats not true from what ive heard... a cracked header will cause a loss of back pressure and ive heard these I-6's are dependant on backpressure
yah it will still run fine, but not at its peak, and long term damage (extreme carbon build ups from the exhaust leaking onto your intake manifold) is always fun
Unless the crack is HUGE, and big enough to make your jeep sound like a garbage truck, it is NOT a problem at all. The loss of back pressure will be very minimal and that will be GOOD for your engine. Carbon buildup will be almost non existant....