My basement drain is blocked. Only my downstairs shower feeds it directly and it overflows. It is not backing up with water flow from anywhere else so I am assuming it is just this U that is blocked. HELP please

By "basement drain" I assume you mean floor drain. If that's so you have a blockage downstream from the floor drain.
Snake the line from the floor drain,(see image) If there's no clean out on the floor drain then feed your snake around the bend in the floor drain. Put out about 10 or 12 feet of snake. Good luck, tom