Rear curtain won't close fully

It's a sign the mechanism needs a cleaning and lube. Temporarily you might try releasing the shutter ( without a lens mounted ) and locking the release button down. Then CAREFULLY from the back, put a little upward pressure on the metal curtain 1/4 inch or so, to twist the curtain slightly on it's shaft. Don't pull to far or you can do permanent damage.

You should have a repair shop take care of it.