code p0720 problem, need car

I have a 2000 Sonata that shows the above code...
What happens is after the car warms up ( it seems that way)and you are driving down the road the car will downshift and the rpms will go up...... It like it bogs down...
You could stop at a stop sign and almost floor the gas pedal and the car will move forward but at only around 20mph ( for example)... or until it gets up to speed but your rpms are higher then normal...
Any help would be appreciated since my other car I have got in a accident....... Thanks

P0720 Output shaft speed (OSS) sensor – circuit malfunction Wiring, OSS sensor, ECM/TCM

And from your description, your transmission might be dead ...

How can I tell if it is the transmission versus a actual sensor whatever that would look like?

either find a complete set of sensors for your transmission (at a junkyard) and change them to see if the problem is still there or go to a garage.