ADC timer definition

[richard]

Then ADCIN is an instruction nobody should use in order to have precision?

why would you say that ?
adcin is only an issue if you use that chip [pic18f4550] with the adc clock option 3 [rc] and fosc > 1mHz
with fosc=20mHz adcin is fine with adc clock option 5

If I use the pins dedicated for AN2 and AN3 in another matters, will the ADC have the 5 volts of Vdd as reference as default? (I hope so because I have any pins left) Or I have to use necesarily AN2 and AN3 to specify that reference?

the default powerup state for vcfg is 00, which defaults to vss/vdd for reference