What does this check engin code mean

[dano82]

I have a 1999 Jeep GC and the check engin light came on. I checked to see what was wrong and the code P0720 came up. When I looked up the code it said:
Low Output SPD Sensor RPM Above 15 MPH

The relationship between the Output Shaft Speed Sensor and vehicle speed is not within acceptable limits.

or

Output shaft speed is less than 60 rpm with vehicle speed above 15 mph (4-speed auto. trans. only).

Now that I have the definition I have no clue what it means, does any one know what it means or if it's something I can fix or the dealer has to fix?