I think of rationalisation as the most effective course of action and justification as the morally right course of action. I think the former has always been a subjective matter. The latter, in the Christian world, has traditionally been viewed as objective but not anymore in our postmodern view of morality.

With both justification and rationalization the person has already decided what he wants to do. The reasons for that desire may not be known, hidden in the subconscious mind.

A part of the person’s psyche though is warning against the proposed action, or there could be resistance to the idea from others.

So there is a battle between the conscious mind of self or others using logical analysis, and the intuition of the subconscious. Both sides of the battle might have some truth. Because you have logically analyzed doesn’t mean you’ve analyzed correctly. You might not have all the pertinent information. Going with the flow is often the best course.

Maybe justification applies to proposed actions while rationalization applies to beliefs and ideas. To justify is to give reasons to the world for doing something while rationalization gives reasons to self for thinking that you want to do the thing in the first place.

In the most simple of terms
an excuse before the fact is a rationalisation
an excuse after the fact is a justification

I like @powder's explanation. I'd take it a step further, though, to say that justification can be abstractions, e.g., whether killing another person can ever be justified and, if so, under what circumstances. I think rationalization is generally made for a particular action where there's a personal stake in the outcome, but if there is disinterest — from others about a specific instance or from one about a hypothetical situation — I think that could be considered to be justification.