The 1999 GM manual has the diagnostics for the transfer case, it may be worth checking out. The two volumes can be downloaded at this link (see post #77)…
Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...
www.gmt400.com
Here is a screenshot example of the tables from that manual…
Regarding your unconnected red plug from your initial post, it kind of looks like the EVO sensor plug on the back of the power steering pump. If the EVO system has been deleted (and they often are as they’re problematic), the plug is just tied up out of the way when the sensor’s solenoid valve is removed. If you don’t have a connector and valve on the back of your PS pump, that’s likely what it is. This is where it would go if it hasn’t already been converted…
Here’s the pump with the EVO deleted: