Am I allowed to sit on a jury that has a case against a fellow Jew in the US? Are there differences in the types of cases one can sit on? What should be the obligation of a Jew towards Jury duty? Sitting in judgement in a Gentile court would seem to violate what I have learned against informing on Jews, judging Jews or helping Gentiles against Jews.

The system using Jury is a forbidden system. One should avoid participating in such a system in any way possible! One should certainly not sit as jury in a court that is discussing life and death. A court should be run by experts not by amateurs, even if experts assist them. Bivracha Rabbi Ya'akov Ariel