[T]he reason liberals today aren't shouting from the rooftops about anti-Semitism is the same as why they weren't long ago: Jews aren't, or more accurately aren't viewed as, marginalized. Being on the left has always been about supporting the downtrodden, and since anti-Semitism is and always was about accusing Jews of being insufficiently downtrodden, there are only these rare moments when the obvious left-wing position is to get worked up about anti-Semitism - moments when anti-Semitism's on-the-ground influence is so great (think the Dreyfus Affair, the Holocaust) that thinking of Jews as victims becomes uncontroversial.
Anyway. Ideally those on the left would see that anti-Semitism is an odd kind of bigotry that surfaces the most when its victims seem to be doing the best, i.e. when they seem the least underdog-ish. And there's no reason understanding this wouldn't be compatible with more straightforward social-justice advocacy, including for the Palestinians.
One of the things I think is important to reiterate about anti-Semitism (or any -ism, really) is that it is inextricably connected to what one thinks Jews are owed. If one thinks the Jews already have too much -- too much wealth, too much power, too much influence, too much security -- then it will hardly seem like anti-Semitism to urge stripping it away from them. That will instead seem like a return to fairness (compare complaints about gays seeking "special rights").
This is in part why anti-Semitism can't be reduced to simple questions about malign intent -- if one honestly believes that Jews only deserve X, but that they have X + infinity, then one won't think one's desire to return Jews to X is malicious. Equal rights, not special rights. Nonetheless, whether that position is justifiable depends on the propriety of X itself -- and that's a normative question that exists quite independent of the good or bad faith of the proponent.