98 Lumina wont move at all in either Drive or OverDrive

98 Chevy Lumina (AutoTrans) wont move while in the Drive Gear. I have to shift down to '1' then up to '2', then at 20 MPH i can shift to 'D' and it will work fine until i slow down below 20 MPH. My Trans Fluid is FULL. If its not the fluid, what could the problem be? (BTW it happened suddenley, i parked for a minute one day, and then when i was ready to go, it was gone.) Thankyou!

Welcome to AF, more than likely the trans is done, you could have a trans shop scan it and check the solenoid functions but it is almost surely an internal issue.