96 CV-PI won't shift

i just got this 96 Crown Vic from a friend. i have had it for about 3 months and i dont know why its not shifting from 2nd to 3rd gear. trans fluid does not seem to be burnt by sight and smell. if it means anything i was using the cruse control when I noticed it. any help would be great.

Depress the button on the end of the gear selector arm, a light should illuminate "OD" or "OD off" (with the key in RUN). If not, have your vehicle's codes read at Autozone.

If it does come on, make SURE the ATF level is full to *slightly* overfull on level ground at full operating temperature.