Assuming that the transmission is OK internally, I would guess that you might have one or of two problems.
1. Would be the vacuum modulator. (more on this if necessary)
2. Would be the mechanical linkage.
Adjusting proceedure:
1. With engine stopped and parking brake applied, placethe transmission selector lever at the steering column in (D) position and hold against the drive position stop ( with a weight of a helper) .
2. Loosen the shift rod adjusting nut (it is on the lower end of the rod connecting to the shift column).
3. Shift the manual lever at the transmission into the drive position, by moving the detent lever all the way rearward then two detents forward.
4. With the selector lever at the transmission manual lever in the (D) position, tighten the nut to 12 - 18 ft. lb. Use care to prevent motion between the stud and the rod.
5. Remove the weight or your assistant from the selector lever ****.
6. Operate the shift lever in all positions to make certain that the manual lever at the transmission is full detent in all gear ranges. Re-adjust the linkage if required.
7. Check for correct operation of the automatic transmission selector indicator, adjust if required. ( If you need the proceedure to adjust the indicator, I'll give it to you in another post.
Good luck,

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