The lock ring is important because it keeps the U-joint in place and keeps it from moving. If you have the wrong lock ring then the U-joint could rock back and forth and cause damage to the the yoke. If you have the outside lock ring it will have a circular shape with tangs and be at the top. If you have the inside lock ring then it'll look like a C and be at the bottom.
Rear Shaft Joint Count
On these models there are either 2 or 3 joints on the rear shaft. You can find these joints simply by crawling under your truck and looking at your rear shaft. However, if you don't want to do that you can also call Dodge and they'll tell you as well.