What is the difference between 后父 and 继父? Is one more formal? Is it the same with 后母 and 继母? My Mandarin textbook says 后父 is used to address your 继父.
They are the same. The only slight difference might be that 后父/后母 tend to be more commonly used in oral communication and relatively less in writing, while the opposite is true for 继父/继母, but this difference isn't really something you should be concerned too much about. They are interchangeable, for all intents and purposes.
They mean the same.
后 is just more informal and colloquial and usually put together with colloquial words like 爹 and 妈
继 is a bit more formal and literal and usually comes before more formal words like 父 or 母
Therefore, it's kind of strange to say 后父 because they are unmatched in terms of formality but 继父 后妈 are good and authentic