the 0/D light is flashing and check engine light is on
check engine light is on on 1998 Ford Mustang
You have codes in your Transmission for sure and possibly your Engine. You need to have a scanner retrieve them and a then have a technician trace them down and NOT chain store parts guy/gal. They are not fully trained to do this. Every day I read about how people waste money chasing after bad info from these folks. They mean well but are not skilled to do this work.