Where do I look for the serial number on a 4th gen rear end to see what the gears are in it? Or can I use the Vin on the car or what number do I need to look for for 3.42?

 

Pass axle, front side. You'll need a toothbrush and some brake cleaner and maybe a brillo pad and scraper to scrape the gunk/grime off to see the numbers. Then you'll have to look those codes up on internet.

BUT - there's no guarantee that will tell you what's inside. Even a 2002 rear end is now 9 years old - that means 9 years for someone to have changed gears inside. The only way to really know for sure is to remove the cover and count the teeth.

You don't really have to count the teeth - the ring will have a number stamped in it, the pinion will have a number stamped in it - divide one into the other.