The shift detent rod is attached/held into the detent mechanism with a hardened steel roll pin. The roll pin is available from the dealer.

To reattach it to the detent mechanism separate the rod from the big square button...you may be able to save the button but chances are you'll need a new one. You'll have to remove the center console cover to get at the shift mechanism and you may have to remove the neutal safety switch to get at the detent mechanism.

Anyway, remove the old roll pin from the detent mechanism and put the detent rod in past the two holes in the detent mechanism. Start the roll pin in one side and use a pliers or pry bar to get the roll pin in past the notch (what you think is a circlip setting is a groove all the way around the rod) and into the hole on the other side.

Put everything back in you had to remove to get to the roll pin and take your old saved big square button or a new one and snap it onto the detent rod. Be aware the big square button goes on either way, but one way the button has a symmetrical relief to the shift selector knob...wrong way and it sits up at an angle!

If this doesn't work you'll have to remove the entire shifter assembly...disconnect the AT linkage under the car, disconnect the OD wire from the solenoid...there should be a connector between the solenoid and the wire (it was yellow on my car), disconnect the torque stay from the shift assembly, disconect the electrical bits from the shifter inside the car, remove the four bolts holding the shift selector mechanism and lift the gear selector assembly out.

You should now be able to get to the roll pin easily.

Since you have everything out, now is the time to replace any plastic bushings that are worn or missing and I would probably replace the OD wire too. The part that goes under the car (through a grommet in the shifter assembly) gets oiled and contaminated and the insulation turns to putty and eventually flakes off. The 87 OD wire on our 760 went about 2 years ago....maybe you have another three or so years.