I'm 99% sure I have to swap the entire final drive ring gear/countershaft, but I just wanted to double check. I'm using an S4C trans with Mfactory diff. I need to stretch the gearing out just a touch and have a good LS trans laying around. Was going to rob the ring gear and countershaft from the LS. I see the part numbers are in fact different between the two, just wanted to make sure that I'm not able to swap my ring gear in and leave the countershaft untouched.

 

No you cannot swap the ring gear only... the counter shaft is a matching set.

 

Even if the part numbers were the same you would need to swap the complete matching set.