That's a great question. If you permit me the equivalent of thinking out-loud, let me see if I can clarify what I mean by "prudential judgments". I think the best way to describe it is that I am referring to the application of a principle to concrete facts and circumstances. Of course, that's not a brightline. After all, you could say that the understanding that abortion is an intrinsic evil is just an application of the principles of the immorality of murder. But just because there is fuzziness on the boundaries, I think it is fair to recognize that there is a large zone in the middle where there are the moral principles and then the application of them to facts.
In that way, I would view the pronouncements on the justness of the Iraq War as prudential judgments. The Church recognizes that war is not an intrinsic evil and therefore the question is left ot judgment about whether a given war meets the criteria necessary to make it permissible. Of course, to say that something is in the realm of a prudential judgment is not to say that that judgment is void of objectivity and certainty. It is not to say it is mere preference. It is to say that judgment and prudence are required.
Consider the principle that torture is intrinsically wrong. I would view the determination about whether a particular act constitutes torture to be a prudential judgment. Suppose someone came up to me and said, "So if I chop of this man's leg in an interrogation is that torture?" I'd answer, of course it is. If they responded, "Well, that's just your prudential judgment," I would respond, "It should be yours, too." Now, I think the degree of certainty on some of these applications of principles to facts can be so high that we by proxy can be relatively comfortable with universalizing the judgment into a rule. Waterboarding is wrong. Or abortion is wrong. But we need to be careful when we do that to make sure that we haven't made a mistake in that universalization. For example, with the "abortion is wrong" principle, one could see how some might take that to preclude the principles the Church does say are legitimate when it comes to a medical procedure to save the life of a mother that has the unintended consequence of the baby's death. Most of us avoid that confusion because we don't recognize that act as an abortion. (And other's manipulate those principles to give cover to acts that are wrong.) My point is that one can have a very high degree of certainty that a prudential judgment is the correct one.
Of course, other areas may be more difficult to decide. What's the right policy to have with respect to illegal immigration? Whatever one might think of the morality of particular proposals on this subject, it is generally an area where I'd hope most would admit less certainty. Thus, my pairing of prudence with judgment.
Returning to the question at hand, about whether Catholics must vote, for myself, I find that I have little basis to justify not voting in a primary. Which is a bit ironic, in that a primary is merely a tool for party members to select their party's candidate. (We forget that because we are used to, due to our two-major party system, the government facilitating that process by operating the elections.) But it is in the primaries where I find myself most capable of finding a candidate that I can comfortably support and need not worry about the classic fear and peer-pressure arguments I mentioned earlier. In truth, if my candidate isn't the nominee, I don't know how I will handle the general election. I'm taking this one step at a time.