My guess is that the exhaust system and DP are in a fight due to heavy load on the tail of the DP. If you are not using the factory hanger [off the tranny crossmember] and you have a big load on the tail of the DP when everything is all buttoned up, this will cause cracks the way you have described. Get an HRpartsNSTUFF d/s engine mount, a THDP and you will have no more of these worries plus have more HP potential.
HTH
Jim