Is revenge ever moral?

It's only 'immoral' if you injure that person. If I go around telling people how my ex- stole my pickup truck, ran over my dog and gave me a case of gonorrhea, and it's all true, the fact it is true means I'm not injuring them and the satisfaction of smearing them with the truth is just a nice benefit. But if it's false, I'm damaging their reputation and injuring them with my lies, and they are allowed to stop the continued injury and to seek compensation for damages due to that injury.

If it's his truck and she just dumps garbage on it, if he can clean it off, you chalk it off to experience and probably try not to do that again. There technically would be grounds to sue for damages for the cost of cleaning - again, no one has the right to injure another - but if all it takes is a $5 or $10 car wash to fix it you probably just figure it's part of the cost of getting caught and forget about it. But if she does $5000 in damages you're probably going to sue. And rightfully so.

No one has the right to injure someone else, but you also don't get the right to duck the consequences of your actions if someone else can make your life miserable as part of those consequences and not injure you either.

So the answer is yes: Revenge which does not cause injury to the perpetrator is not only moral, it's probably the best fun you can ever have, presuming you find that sort of thing fun. If, as in the previous example, someone has a truck with no way to clean it, and you - without scratching the vehicle - clean the dirt off so it has in very clear wording 'I cheated on my girlfriend with her best friend and her mother at the same time' and it's true, it's probably going to be very amusing to consider how they're going to have to drive around with this posted for everyone to see.

(I don't know the song, this is just an example.)