Nope, no prune juice in Dr. Pepper.
Check out the site that yamahaXS linked earlier and check out the ingredients.
From the site: What?s in Dr Pepper? Is Prune Juice in Dr Pepper? What?s in Dr Pepper? Is Prune Juice in Dr Pepper? We don?t know what is in Dr Pepper. The Company holds the formula in a vault. The Company will, however, tell us what is NOT in Dr Pepper. Prune juice is not and never has been in Dr Pepper. That is an old story that has apparently been in circulation since the 1930s, when some delivery man of a competitive drink may have said, ?You wouldn?t want to drink a soda that tastes like it has prune juice in it, would you??