Justice is subjective, after a fashion.

An intersubjective fashion. It's not one person's preference. If justice were objective, Socrates should have tested it in the laboratory instead of discussing it. If it were subjective, he needn't have invited his friends over--he didn't need them to tell him who makes his favourite restina. Justice can only be intersubjective because it regulates interactions among people, and the appropirate way to decide intersubjective issues is to solicit a range of opinion from a number of people and iron out the bumps. I don't see anything broken in what Socrates was doing. We still do it, in the forms of ethics committees, think tanks and panel discussions.

It is a set of intuitions, systematised into commonly accepted laws, which can change over time. Whether people are in jail isn't part of the phenomenon 'justice', only of how people act on these subjective ideas.

I can't see what distinction you are drawing. If there is s a phenomenon of justice, it is an intersubjective way of combining preferences that fulfils certain criteria, such as being the same for all, in order to regulate certain concrete events, such as who lands in jail. So who lands in jail is in fact part of the intersubjective idea.

On the other hand, people can be rightly-imprisoned-for-me and undeservedly-imprisoned-for-you if we disagree about the law.

I don't see why. If I think 2+2=5, then "2+2=5" isn't true-for-me, it is just wrong. Disagreement is not a sufficient condition for something's being properly subjective.