The first thing you have to worry about it is the input shaft on the transfer case. The 1996 AW4 will have a 23-spline output, but the 1990 transfer case will have a 21-spline input shaft. Another factor is that gear cut on the input shaft changed around 1994, and there are different lengths for different transmissions, so altogether you will need an input shaft from a 1991-1993 NP231 or NP242 that was originally behind an AW4. Then you will have to swap it, which requires complete disassembly of the transfer case all bearings, so you might as well get a master rebuild kit and replace all the bearings and seals while you are at it.
The other thing you have to consider is the rear driveshaft length. The rear axles are the same so pinion length is the same. The rear output on the transfer cases are different lengths but if you have the rear driveshaft from the 1990 transfer case then its irrelevant, just remember to match the driveshaft to the transfer case and you will be fine.
Probably easier and cheaper to just buy a new transfer case from 1991-1994 and swap it along with the 1990 rear driveshaft. If you get a transfer case from 1995 through 2001 you will also need a late-model driveshaft to go with it.