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.
2 more answers , 2 more comments
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.
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