1 Cor. 12.13: For by one Spirit we were all baptized into one body, whether Jews or Greeks, whether slaves or free, and we were all made to drink of one Spirit.The relationship of the two formulas is clear not only from the Jew / Greek and slave / free pairings but also the fact that both have an explicitly baptismal context. But where Gal. 3.28 has "there is neither male nor female", this is absent from 1 Cor. 12.13. Now if 1 Corinthians was written after Galatians, Paul has taken out the male and female phrase, perhaps in light of the issues in Corinth connected with women in worship (1 Cor. 11.3-16; 14.33b-6). In Heretics, Lüdemann calls this a "treacherous" move, if I remember correctly. So here is an advantage if Galatians post-dates 1 Corinthians: Paul's addition of the more liberated "male nor female" represents his more mature, developed thought, not something that he dropped because his attitude had changed.
Gal. 3.27-8: For all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus.
But of course we do not arrange Paul's letters in a particular order so that they become more palatable for us. The question is whether the historian can imagine why Paul might have added the phrase in Galatians? As it happens, there is a good reason. Galatians is all about circumcision. Paul is insistent that Gentile males in Christ do not need to be circumcised; indeed, to accept it is tantamount to turning to another gospel. In such a context, one of the issues relates to women. The discussion in Galatians is all, typically, a very androcentric affair, but what Paul sees in 3.28 is that baptism, unlike circumcision, is an initiation rite that involves women as well as men. Perhaps it was this context that caused him to reformulate the statement. I would like to think so.