1998 Toyota Camry shifts hard

98 Camry runs good considering the mileage. . But engine light has been on steady for a few months. I don't have the engine code but the guy at the parts store said it was a oxygen sensor that needed to be replaced. Sometimes I have to keep one foot on the gas when stopping, to keep the rpms around 800 or it will die. No big deal but recently it has started shifting hard. It goes into the gears fine but when it shifts into second and third you can feel a pretty pronounced sideways tug. I assume that is the transmission. I checked the transmission fluid level and it is ok. I don't think it has been serviced in a very long time. I really cant afford to replace the transmission. Are there some cheaper fixes I can do to get more miles from the transmission?