Just my opinion, but unless the crack is very large or near a bolt hole I'd ignore it. What you are likely seeing is a typical Jeep oil leak. First step is to get a finger full and decide if it is engine oil or tranny oil. And then keep an eye on it to see how much it actually drips on the ground. A little oil can look like a lot, but may just be seepage. Seepage is usually defined as a wet spot that doesn't drip much if it all. Check your engine oil and tranny oil frequently and see if you are in fact loosing any significant amount. I always check all my fluids on a new to me Jeep, no telling when the last time anybody actually physically checked the transfer or differential oils.
Oil filter adaptor, rear valve cover, front and rear main bearing seals are all spots to check. Transmission cooler lines are a common place for leaks, rear transfer output is also a common spot for leaks.
The bell housing isn't sealed, it is just a cover for the flex plate and torque converter or the flywheel and clutch. It does mate up the tranny to the engine, but really isn't prone to significant damage.
Oil often blows to the rear when you are driving and the oil you are seeing may be coming from in front of the transmission all the way from the front of the motor. Most times (often) it is a leaky rear main bearing seal, they are prone to fail. Quarter sized leaks on the ground I ignore, saucer sized leaks need some attention.
