There are two linkages going to the transmission. One is the kickdown lever from the carburator that controls the downshift when you let off the throttle and the other is the linkage that comes from the column shift.

What may have happened is that the linkage has slipped or is worn. Put on your parking brake, and slide under the transmission to find the lever that controls the shift position. Loosen the nut and move the lever on the transmission firmly in the park position (furthest forward) while at the same time holding the column-shift lever firmly in the PARK position and tighten the adjusting nut.

WHILE THE PARKING BRAKE IS STILL SET, start the car and move it through the gear positions. It should feel solid in each one. If something strange happens (like the R position on the column becomes PARK or NEUTRAL then loosen the adjusting nut and re-adjust the linkage, you'll get it right!

I have long since converted my '76 two-door to a B&M StarShifter floor mount and use a Lokar C-4 kickdown cable to control the downshifts. Works GREAT!

Regards,
Tracy