What can fix this transmission problem? on 1996 Cadillac DeVille

I just got a car that has transmission problems with 135k on the car. If you put into (D) Drive, the transmission will quit engaging at around 45 mph. If I shift to gear position 3,2,1 the transmission seems to function normally.

I have been told that the transmission needs to be replaced, but I am skeptical. Any suggestions would be appreciated!

Asked by for the 1996 Cadillac DeVille
on 1996 cadillac sedan deville po717 I had over fill the transaxle so I remove some of the fluid and it started to shift into high gear.
3 answers 2 comments
Check for trouble codes.If you have PO717 code (input speed sensor)you will not get you'r 4th gear.You do not have to pull the trans. Lower the engine/trans. cradle just far enough to get the side pan off through the left front wheel well.after removing the pan you will see the sensor /wires when replacing it you should get the distance specs between the contacts.Any way its a lot less expensive then removing the engine and tranny. Good Luck JRHardy
seek diagnostic at Aamco transmission
Not the answer I am looking for...Thanks!
Why pity? Your transmission is not engaging at higher speeds, if your fluid level is o.k....your transmission is bad.