Why can't all four of my parents just shut the hell up!

You don't have to accuse him of anything. Just point blank tell them that you really feel uncomfortable about discussions coming up with the other parent involved. You wish to be kept out of it - good or bad. That it makes you feel uncomfortable because you love them all.

Tell them that you are the *kid* and it is wrong and unfair of them to put you in the middle of their adult problems. Not that you are a kid, but it might get them to put things in a little better perspective.
Also I agree with your mom that your step mom can't be trusted to not tell your dad everything. I won't tell her anything that I didn't want getting back to my dad.

I am sorry I didn't see this thread yesterday. BiW, I had the same problem when I was your age and it carried on into adulthood.

I tried this tactic when I was your age first. I remained very calm and quiet and looked them straight in their eyes and said: "I am going to tell you what I tell her (him), she is my mother (he is my father) and I don't want to hear it. Your issues are between each other, not me. I love you both equally but please leave me out of it." Then I would hang my head and slowly go up to my room. You would think that would work but it didn't completely stop it. My parents were very angry with each other and totally self-involved.

Finally when I hit my 20's I found the perfect solution for myself. I would completely ignore what they were saying and talk about something completely different. You could just talk about a problem you are having in school or with a friend that you can pretend you need them to impart some important parental wisdom on. You can talk about a homework assignment you have to complete and ask for help. Talk about a conversation you had with one of your grandparents -- the parent of the person you are speaking with. Talk about something funny that happened with your brothers, your friends, an incident at school. Talk about something that was in the news and encourage discussion about it by asking questions. Ask their advice about something. My point is, just arm yourself with enough subject changing material ahead of time so that you won't be floundering for a topic change. Usually a funny story or joke is a good way to take their minds off the negative topics. Either they will go along with the change of subject, or question you why you are changing it. If they question you, then use my first tactic that I used as a teenager that I mentioned above. If you can muster up a tear in the eye while quietly telling them how you feel, neither one will get angry with you. How can you get angry at your kid who is crying and hurting? It should really make them think about what they are doing to you. If either one responds and talks to you about it, then you can go on to tell them how upsetting it is for you to have to listen to him/her bad mouth your mother/father. You could tell them Dr. Phil says it is the worst thing a parent can do to a child. The problems between parents need to stay between the parents. Then stop talking and let that sink in.

Just come right out and say it... SHUT UP! That's the way, it works when my parents are fighting.