If the shifter won't go back into park on your 1996 Buick LeSabre you may have a problem with the shift cable or its adjustment. The shift cable will need to be disconnected at the transmission in order to check for proper operation of the cable and transmission. With the cable disconnected at the transmission try moving the shifter inside the car - see if it will go into park, also manually shift the transmission to see if that too will shift into park. If you find a problem with one or the other then that issue must be repaired. If both of the above tests are OK then check the cable adjustment, if that is OK then chances are you will need a shift cable.