meh i dunno, i can be pretty harsh with comments and stuff, but i am also pretty free with my compliments and i don't touch the stuff that i know really upsets them. I think the fact that i am willing to joke about the stuff they see as "bad" or "wrong" with them means i dont find it that bad, and because i dont ignore it, it means when i do the opposite and give compliments, they know its genuine and it really means something.
I do the same thing as you with regards to friends, if someone wants to just vent because a friend is pissing them off thats fine and i can listen to their problems and try and sort it out, but in a group situation when its like a bitch fest i just tell them to either go and sort it out with that person and shut the fuck up in the meantime, or i go can hang out with others who arent badmouthing the people i care about.
Sometimes in a group of friends, it starts to get like one person cops a lot more shit then everyone else, and when i realize i am doing this i hate myself for it. Even though they know its just joking, you cant just pound on one person continuously without some sort of negative consequences.
Thats probably whats happening to you maybe they see you as the weakest one, and to boost their own egos they give you shit because you are the easy target. If you tell one of them that it hurts you and maybe you can pick on each other equally then word will spread to the others and maybe they can act with a bit more respect, especially over issues that are really sensitive to you.
Then again, maybe they are cunts. In my group of friends we might joke about stupid stuff like the other week on of my friends was buying stuff on Ebay and asked me what "AUD" meant, for the last week, every time anything about money has come up like how much the hot chips i bought us for lunch cost, i would answer like "$5.00 AUD" thats like funny teasing, but even then i am wary because i dont want to fall into the trap of just making jokes that give the message like "LOL YOU ARE DUMB LOL" because that can damage self esteem.
The types of stuff we wouldnt joke about are like " oh hey, the place we are moving into only has one parking spot and iam going to use it, is that ok? oh wait, you dont need a spot, your car got stolen LOL" well thats kind of funny when put like that, especially if its an accident, but the harsher ways like " where did you park your car? oh wait it got stolen HAHA" we dont do. I guess its just about moderation