If the exhaust manifold is cracked, get it fixed very soon. Its also a safety concern. An exhaust leak in the engine compartment can get into the cabin from the fresh air intake below the windshield. And you can't smell carbon monxide...bad to breath.
If the manifold is cracked bad enough, it will cause a rough idle. I would also check the ignition system for problems.
When you have your manifold replaced, use either a new one from the Jeep dealer or install a Banks or Borla header. The aftermarket "Cheep" manifold are just that ....Cheep. I have installed a number of them, They usuall come back .. cracked.
[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.