There is a rod fitted/connected that extends from the gear shifter to the transmission gear lever. The rod has clips that hold it connected and fail with age.

Drive the car up on some ramps, and with the emergency brake on, tires chalked, (safety) crawl under and and look (drivers side) up between the transmission and transmission floor hump and you should see this rod, about 12 or 15 inches long, 3/8" diameter. have someone move the shifter back and forth. At this point, it should become clear if the shifter rod is moving the transmission lever located on the drivers side of the transmission. You should be able to tell right away if something is amiss! Hopefully it is just this simple and all you need to do is reconnect the rod and clip that holds it in place. If not, it could be something wrong internally with the tranny........good luck...BB