Try changing the transmission filter and fill with new fluid. It sounds like the transmission is not shifting gears. Remember automatic
trans operate using hydraulic pressure that is built up inside the
trans to shift gears. If the filter is dirty or clogged no or little pressure can build up.
Unbolt the pan on the bottom of transmission and the filter should be right there. They should be changed, I think around every 60K-100K not sure though it just depends on the vehicle.
Filter kits are around $20 at autozone.