It is technically slang as the correct grammatical structure would either be adding the information in an adjunct phrase "He ran while naked." or using the adverb form of naked instead "He ran nakedly." However, as randy has pointed out, more and more people are doing what I call "adverbifying" adjectives. This can be seen very commonly in the increase of people saying one does something like " I'm doing good." instead of "I'm doing well." Hope this helps :)

For absolutely correct grammar, it should be "nakedly". But few people would actually say that.