is that possible making the Vref low = 2.5 and making Vref High = 2.6 , meaning every 1mv equal 1 count, regardless the resistors errors, and noise
or the capability of the LJ12 isn't enough, i tried to find this info. in the DataSheet but i failed;
Datasheet sections 15.7.3 and 15.7.4 (MC68HC908LJ12/Rev2.1) want you to connect Vrefh to Vdda and Vrefl to Vssa.
Specifications (Table 23-10 and Table 23-11) seem to imply any value is possible for Vrefh and Vrefl as long as they are within Vdda and Vssa, respectively. (Personally, I don't think you can set Vrefh and Vrefl to be only 100mV apart; the above sections seem to be more accurate than the tables.)
Even if you can do it, you won't have 1 count per mV, if I understood you correctly. 1024 steps in 10-bit A/D, 100mV (2.6-2.5V) give a resolution of ~98uV 256 steps in 8-bit A/D, 100mV (2.6-2.5V) give a resolution of ~391uV