Nieboszczyk is a bit obsolete to boot and trup is very colloquial - if for example someone mentioned something about a trup while referring to the dead body of your family member, you, being Polish, would feel offended a bit then and say sarcastically that it's good at least that this person didn't refer to the body of your relative as a carcass. Because the most proper term for it is zwłoki - a corpse 