You could ask the bailiff to bring this concern to the attention of the judge. If you feel the need to protect yourself, you should. If you are really sincerely scared, stand your ground and refuse to participate for this reason in your response to the judge, even if this puts you at risk of a contempt of court sanction - better to being in jail briefly than to be physically harmed by another juror. The judge might release you from jury service or dismiss the threatening juror, or might declare a mistrial if you bring this to the judge's attention.
The mistrial could result in the de facto acquittal of the defendant in a criminal case, especially if no alternates are available and dismissing you or the threatening juror would bring the jury below a quorum, so keep this in mind.
I would also not tell the bailiff what position you are being coerced to take, merely that you are being coerced with physical threats. And I would identify the threatening juror (who might also be removed).
Of course, if you are actually on a jury right now, you are almost surely violating your obligations as a juror by going on line to ask this question.