just as the title saids. does anyone know how to lock an open differential

Weld the spider gears together ! This isn't suggested as an option for a street driven vehicle - race track only ! It's fine for straight ahead driving ( I.E. - drag racing ) but it's a different animal when you have to turn corners . A more realistic option is a limited slip differential which is fine on the street . Just my $ 0.02 ! Good luck !