PIC 18F: Can A/D (+) Reference voltage exceed Vdd?
The analog reference voltage can not be greater than the supply voltage.  

Have a look at the datasheet of PIC18F4550.  Table 28-28 on p.404 (param no. A21).  Vdd is the maximum.



Strangely, the datasheet for your particular PIC doesn't seem to have a table for A/D, although it has got a table for D/A (table 29-12 on p.463).