105 out of the factory is full time 4WD, but there is the possibility to modify the transfer case and install free wheeling hubs on the front axle - there's kits (Google MFK1210-100HD) available for same. Most positive reviews on these kits are where the vehicle never goes off road again..... You certainly won't get a lot of extra fuel economy.
The 100 series refresh in 2003 did f all to the 105, fuel tank changed and as I remember OBDII ports were introduced - these had previously only been available on the 100. I haven't yet run into a part from 2003+ that doesn't fit my 2001.
Cheers,
Rock