Ok, this is a meaner method of instant que and assumes one of you isn't a tank. Get a group of three together, one person being a tank class. Have him sign up as a tank (even though he isn't). When the three of you get in an instance vote kick one of the dps, when it asks you to choose your roles have the "tank" choose only damage. You should get a new tank quickly by sacrificing a patsy.
I don't know if this is a repost, my apologies if it is.