I have a 91 yJ 4.0L high output with around 100k on it, the previous owner told me he bought it with 40k on it and never did anything to the engine but change oil, filters and plugs. As far as I can tell it's the original manifold. It's still going strong with no leaks or cracks. I had to replace everything behind the Cat exhaust wise though....
I think they tend to crack just above the flange. Look for signs of extra welding in that area. The pipes are steel and welded together instead of cast iron so if you see a weld that looks crappy or not where one should be, It's been fixed, maybe!