Am I obligated to continue to work on a paper I contributed significantly to?
I think the Golden Rule applies here. While you have no obligation to act against your own self-interest here, if you were in your former collaborators' position, you would probably be rather unhappy at being blocked from publication because one co-author is no longer available.

Is your preference to not work on this paper at all, or just to limit the amount of time you spend on it? From your collaborators' point of view, a few hours of your time might be  sufficient to avoid blocking the publication, while not placing much of a burden on you. It's perfectly OK to tell your collaborators, "I'd like to help, but I have many other demands on my time right now. I can devote 4 hours this Sunday to helping you get this publication ready, please let me know how I can use this time in a way that is most helpful to you."

On the other hand, if a few hours is really unmanageable to you right now, just tell them this, and let them know if you expect to have a little more time in the near future.

I suggest that you learn from this experience going forward, and make sure to document your work in sufficient detail for others to follow before you leave any research project.