Hello Juan,
For the K66, the ADC0_DP0 is not available for differential inputs. Only the pins 29 and 30 can be used as differential inputs and are shared between ADC0 and ADC1.
The inputs are ADC0_DP3 and ADC0_DM3 for ADC0 channel 3 and ADC1_DP0 and ADC1_DM0 for ADC1.

Best Regards,
Gerardo