Elevate the tail of your car from ground, and change the shifter switch to the forward and high speed position. And press gas pedal, inspect the two rear wheels to see if there is one rear wheel turning or both rear wheels are turning.

If only one rear wheel spins, it means that the gearbox is no good; you need to replace the gearset. Click here for detail.

If the gearset is good, you may have a bad shifter.