A/D conversion problem with 18F2455

[skimask]

But I think can figure this out with VREF+=AN2 and VREF-=AN3 by pulling AN2 to +2,5 volt and AN3 to -2,5 volt.

Good luck with that AN3 theory and hopefully you've got a couple of extra 18F2455's laying around.
Reference the 18F2455 datasheet, Section 28.0 and Table 28-28, Parameter A22.