I have replaced a number of them on the 4.0L, all seam to crack ing the same area. Usually around the collector or rear cylinder pipes. Cracking is a real problem.
I would either go to a Borla or Banks header. If they are out of you price range. I would go through the dealer for the "improved" version. You can look around on the web and get a lower than retail price on them. They can still crack, but it was susposed to be somewhat corrected. I have installed a few aftermarket versions, I did not like them at all.. and will not use them again.
While your in there replace the fuel injector seals and clean the bore around the injector. I have found them to get coked up .
[FONT=Trebuchet MS]01' Jeep TJ (Jeep #5)
3.5 RE Custom SuperFlex, Raingler, Kilby, Currie, Banks, Borla, American Racing, Custom mods, Always more in the works.