On a top loader 4 speed like you have, 2 main things I've found that get loose are a worn retainer pin, (1/4" drill rod), or the retainer cap that holds the spring and shifter itself gets loose. Remove the cap, it's threaded, and see if the pin is worn. The pin is on the driver's side of the shift tower. Then make sure the cap is on tight. Everything is under the shift boot.