I believe the body and frame rails are the same, however the transmission crossmember must be moved, and the front suspension mounts may be different.
I also believe the Gen1 Rangers only offered 4wd behind the V6 engines, so the Mitsubishi FM transmissions that will support a 4wd transfer case won't bolt to the 2.3l block. So, you'll either need the V6, an automatic transmission with the correct bell housing, or some funny transmission solutions.
And of course, you'll need the entire front suspension (including front drivetrain) and hubs, the transmission, transfer case, and shorter driveshaft, and whatever parts are needed to make the rear axle gearing match the front.
If you want to end up with a stock-ish 4wd, you'll need so many parts that it would probably be easier to swap your existing body onto the 4wd chassis, than try to move all the 4wd parts into your existing truck.