Is relocation assistance offered to postdocs?
It varies. In my experience some of the more prestigious postdoctoral fellowships sponsored by universities might have some amount of moving expenses built into the budget, and more typical postdoc positions funded out of grants probably won't. But it might be possible to get a professor to help you out, perhaps out of startup or some other funds that aren't earmarked for a particular purpose. You should try to negotiate it with the person who offers you the job before you accept the offer.