Not sure what is going on with your setup, I do not see the problem.
I dug out a board with an 18F2550 and ran the below code to make sure it works. I only have the ADC on channels 1 and 2 as I have a LED soldered on channel 2. Other than that it should all be the same.

Code:
'<FL_PIC18F2550>'
    '<FL_PBPL>'
    DEFINE OSC 48
    #CONFIG
     __CONFIG   _CONFIG1L, _PLLDIV_1_1L & _CPUDIV_OSC1_PLL2_1L & _USBDIV_2_1L
     __CONFIG   _CONFIG1H, _FOSC_HSPLL_HS_1H
     __CONFIG   _CONFIG2H, _WDT_OFF_2H & _WDTPS_512_2H
     __CONFIG   _CONFIG2L, _PWRT_ON_2L & _VREGEN_ON_2L
     __CONFIG   _CONFIG3H, _PBADEN_OFF_3H & _MCLRE_OFF_3H
     __CONFIG   _CONFIG4L, _LVP_OFF_4L & _XINST_OFF_4L
    #ENDCONFIG
    CNT VAR BYTE
    ADCON2.7 = 1
    ADCON1 = %00001101
    TRISA = %00000011
    DEFINE ADC_BITS 10   
    DEFINE ADC_SAMPLEUS 50
    ADC_0   VAR WORD
    ADC_1   VAR WORD
    
    START:
    CNT = CNT + 1
    Serout2 PORTC.6, 16572, ["TEST ",DEC3 CNT, $d, $a]
    Serout2 PORTC.6, 16572, ["ADC_0 ",DEC4 ADC_0, $d, $a]
    Serout2 PORTC.6, 16572, ["ADC_1 ",DEC4 ADC_1, $d, $a]
    TOGGLE PORTA.2
    PAUSE 1000
    GOSUB GET_ADC
    GOTO START
    
    GET_ADC:
    ADCIN 0, ADC_0
    ADCIN 1, ADC_1
    RETURN