I have the 3.21 in mine, but I have the 318 and a 5 speed... a while back I emailed Daimler Chrysler and asked for a build sheet on my truck -- that's how I found out what mine was. All you have to supply them with is the VIN and they'll send you the factory build sheet with every option and configuration of your truck, including the rear end. Here's the website you need to go to:
click on "send us an email" then fill out the info, and in the "comments" box at the bottom, tell them you are interested in obtaining the factory build sheet for your truck. You'll hear back from them in a week or so!