When upgrading gears in a JK D30 that did NOT have the factory 3.73 gears you need to use the YCD706008 and the following small parts.
2007-2011 you can swap the JK spiders over and use the side gear thrust washers from a TJ. YSPTW-014
, 2 required.
But 2012 and later you will need spiders YPKD30-S-27
and carrier (YCD706008) as they changed the dimensions in the JK gears and carrier once again.
This is a cost effective way to get the 3.73-5.38 carrier to work in your JK. The factory JK style 3.73 carrier only comes from Chrysler loaded and is $415!