It sounds like the transmission is in "limp in" mode. When a trouble code is set in the computer, the transmission goes into "limp" where it will not shift. On that vehicle, a common cause of the problem is the transmission output speed sensor.
Have the computer scanned for codes (the transmission is scanned using the blue connector under the dash) that will tell you what the problem is. If it is the sensor, it is not an expensive repair, it just screws into the transmission.
29 years in the auto business