[Big Red]

If your warranty is still good, take it back to the Dealer. If not, and you know a real good mechanic, it's time to buy their lunch. Rattles and leaks can drive you insane.
You might want to check the u joints.
Big Red

 

[Big Red]

They are not suppose to be loose. Put in park, crawl under, now at the front of the try to turn the drive shaft and then do the same at the rear where it joins the rear end. You also need to try to push it up and down. There will be a little play when you turn it side.
From your post I recommend you take it to a good mechanic.
Big Red

 

[Big Red]

The truck should have had a 5 year 100,000 power train warranty on it. You need to check with Ram and see if the warranty is available to you and how much would it cost you. I don't know what the rules are. It's worth checking out.
Big Red