2014 Altima exhaust question
 

Just turned 50k miles and noticed a vibration and louder than normal engine noise occasionally when accelating on a recent trip. Went to dealer and was told there was a TS bulletin for fixing an exhaust system vibration that csuses the vehical to shudder. They installed shims?that are suppose to make the exhaust pipes more rigid. Was also told the exhaust pipe from the converter to the muffler had to be replaced because there was a crack in it probably caused by the shudder. $900! My mechanic put the vehicle on his lift but we couldn't find any crackes, holes etc. There is some sort of metal braid as part of the pipe but can't get into it to see if this is what they are talking about. Has anyone run into this or heard of this problem? I cant find a TS on this anywhere.