mine is doing the same thing. mine was the converter dodge is known for this. summit racing has a good price.

 

Mytruck sounds like it has a bunch of marbles rattling around at low speed or idling and it seems to becoming from the converter. Is that the type of sound you are talking about? Perhapsyou are just noticing it more in reverse because you're going slow or looking back. I got 75,000 and I'm thinking of changing the converter out.