Problem Reading PIC18F4550 Input
I don't think ADCON will make a difference in this case seeing PORTD pins you're using are multiplexed with Enhanced PWM Mode and the Streaming Parallel Port (SPP), rather than Analog Input Module, which take priority over port I/O

You'll want to disable the Enhanced PWM that're also multiplexed with PORTD pins. 
Also you'll probbably want disable SPP, by clearing SPPCON register bit 0 (SPPEN) to 0



So a solution in this case can be to configure CCP1CON to anything but Dual or Quad PWM by setting CCP1CON to 00xx 11xx