you need to change the mainshaft in the trans or find a 2in style input gear for the transfer case. You need to swap output housings in the back and you need to remove the solinoid from the top of the t-case and you need to swap the front flange for the smaller D90 flange or use a DII driveshaft.
All and all $$$$
see my ad in classifieds for a correct D90 case or find an early disco I case (i have one of those too)
you will also lose difflock if the t-case is from mid 2001-2003