Help with 96 Celica A140E stuck in gear?!

[Emacha]

Hi all, my daughter brought home a '96 Celica GT from an auction. Runs great, but trans is stuck in forward gear in all selector positions. (park, neutral and reverse, car wants to go forward) Would like to know where to start looking into this? I pulled pan off and mechanical linkages inside work normally. I am wanting to know where problem may lie? Electrical? Torque convertor? Valve body or internally within the clutch packs? Have rebuilt autos before, but none with the amount of electrics or a lockup torque convertor before? Not quite sure where to start with this and don't want to spend money on throwing parts at it? Thanks in advance any help you can offer!