Have you checked the speed sensor and its wiring? Didn't see that mentioned yet. And, no I don't really know how to test the sensor itself, maybe spin it with a drill and hook up to multimeter. If I recall correctly the sensor connections at the control box are similar to a phone plug (landline), could be bad connection. Good luck.
I'm not an expert, but I installed one on my brother's truck 20 years ago. The control box is sort of like a Murphy switch or safety switch. It can have power but you can't shift until certain criteria are met, auto or manual shift mode, 4wd drive lock out and 20-25 mph forward speed. Once those are met it sends power to the shift solenoid on unit. The gear vendors can work with just a single power switch to the solenoid, but then the driver is responsible for the safety of the driveline. If you run it below 20-25mph you'll burn up the internal clutch pack because not enough oil pressure in unit, bad. With 4wd lock out the front and rear axle will have different effective drive ratios, bad. Auto and manual shift mode you don't want the unit shifting automatically with a manual transmission as you'd have a huge driveline shock, bad.