2004 chevy blazer 4x4 needs starter dont know which one it is without taking it off

Just ask for it the year and engine size. Other way to findout is by calling ur local chevy dealer, they'll ask for the vin# once they find it, ask fir the part # and ur local auto parts can cross reference it

There is only one, you have a 4.3. They may give you an option of gear reduction or direct drive but they both interchange. Gear reduction was an upgrade from direct drive.