DML FAQ : Transmission and Gears : Why do I get a clunk when shifting my automatic into reverse, and can it be fixed?
Dodge considers it normal. It's most likely slack in the drivetrain being taken up. Workaround (or whatever you want to call it): When you park, put the truck in Neutral, apply parking brake, release regular brake, let it rock, then put it into Park. When you go to drive it next time, the clunk won't be there because the slack was taken up previously.