The shifter is nothing more than a lever that engages a cable that pushes the shift lever on the transmission into the selected gears. Starting at the full forward position, Park, reverse, neutral, Drive, 2, Low.
It is under the console cover on the transmission tunnel and can be accessed by removing the carpet on the driver's side of the transmission tunnel
The shifter has a rod that goes up the center of the handle and is spring loaded. The spring causes the rod to push up on the button on the shift knob. At the bottom of the rod is a latch that engages in the pivot of the shifter. On the pivot is a metal arch that has a series of steps in it that engages the latch on the rod. These are what keep your gear selector in place! With the selector in Drive, you should not be able to downshift without pushing in the button on your shift knob. With the shifter in neutral it will go into drive without pushing trhe button , but not reverse or park.
So, bottom line is, if the shift latch is not engaging, it will not latch.
If the spring is broken, it will not latch.
If the rod is gummed up with old grease, it will not latch.
If the steps on the ratchet are worn round, it will not latch.