my 94 cutlass supreme just stopped backing up

What is your question?

was wondering why my tranny is acting stupid. it goes into 1st 2nd 3rd and overdrive fine...but wont back up...any help out there for this? its like reverse just quit, and its an automatic overdrive tranny, does anyone know which others will work in it and why it wont back up...got 173,000 on it but otherwise no problems at all...please help