your Jeep has a two piece exhaust manifold. replace it with another factory unit, grab it from the junkyard if you have to. Wrap both manifolds with DEI Titanium header wrap & spray them with header wrap sealant.
the two piece'rs aren't supposed to crack. anything one piece, with the tubes from cylinder #1 welded to #6 somehow will crack...and quickly.
the 4.0L's head is big, and long. it likes to move from thermal effects.