If you have the push button 4wd like in the limiteds it's super easy. There is a blue plug on the driver side of the t case with 4 wires going into it. This is connected to the sensor that tells the ECU if the tcase is in low. Disconnect this plug and it will think you're always in high range so the 4wd button on the shifter controls 4wd in both high and low range.
I just did this on my 98 a few months ago and it's working nice so far. There are a few threads on the topic and iirc the j shift t cases are a little more difficult to do it to.