Well easiest thing for you to do would just be to call down to the municipal building, but when I lived in South Brunswick we had one up for about 6 years and didn't get harassed about it once (we also lived in the west bumble**** portion of south brunswick though) I know in Spotswood there are a few houses that have had theirs up for atleast two years.
Closter probably has a book about as thick as your house full of "Borough Ordinances" Which are just silly small town rules that aren't really laws, however the town has approved to enforce them. Portable garages could be one of those, otherwise there's no law that says no.
Here's the FAQ on portable garages, like it says on there, no rule that says you can't but some municipalities might now allow it, and since you live in a smaller town, you might be screwed, but I doubt it:
http://www.portablegaragesandshelters.com/questions##13