Not a crazy idea at all. In fact it's the first part of my 4x4 conversion.
You'll need a 4x4 transmission (of course) and t-case (of course), and a new rear driveshaft.
You can either use a whole donor transmission, or convert the back end of your current transmission. This consists of swapping out the output shaft and rear housing of the transmission. This goes for pretty much any Ranger/Explorer transmission. Each shaft is specific to each transmission type, auto trans stuff won't work with manual trans stuff, etc. The transfercases all physically swap though.
For a 2wd with a transfer case I recommend sourcing a BW 1350 transfercase, they are MUCH easier to find than the BW 1354 in a manual version. And you don't have to worry about the front driveshaft yet anyways. You may need to shift it from underneth the truck, some of the linkages don't always match up.
If you do go for a electric BW1354 make sure you get the entire donor harness and module. The harness will run under the drivers seat and the module I'm told is behind the radio/HVAC facia plate. Manual BW1354's go in the same as the 1350's.
Source a stock rear driveshaft from the 4x4 version of your truck's body style. Standard cab/short bed, standard cab/long bed, or Extended cab. Wheelbases are similar enough through the years it shouldn't matter too much. You may run into u-joint size differences through the years, but those topics have been covered many times. Search for "u-joints" and "flanges" in the Ranger axles and 4 wheel drive/transfercase forum sections.
That should get you pretty well on your way!
Welcome Visitor! Please take a few seconds and 
Listen to him.
