Mailmen are a special case (because the postal union pushed for the special case), but Post Offices are generally part of the state in which they are located, and if a murder occurs between civilians (not mailmen), it is tried in State court. On the other hand, certain (not all) military installations are not considered part of the state, and that's why the military and federal law has jurisdiction within the military installation. An interesting crime occured near here several years ago where the perpetrator fired a gun at a passing car outside a military post, missed, and the bullet went through the militay post fence and hit a child living on the base. Because the shooter was outside the military installation when he committed the crime, it was tried in the local State court.
Regarding the 5th Ammendment, skipping words tends to change it's meaning (like the Libs do to the 2nd Amendment). The fifth was written to protect the rights of criminal defendents, let's not change it's meaning:
"No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb, nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use without just compensation."
Also, while your anecdote may be interesting (to you), you're still evading the issue. Clearly in this country there are instances where a person could be murdered and this crime not be covered by state law. According to you, this crime could not be prosecuted, because any federal law against murder is unconstitutional, yes?