In the past, I've tested 4x4s by locking in, then turning lock to lock while slowly moving forward on loose ground. Then I could feel the front tires grabbing as they turned on different radius.
I test drove a 90 GMC today, but didn't feel any grabbing when I used the above approach. So I tried goosing it in loose gravel while in reverse / 4 low. Didn't see or feel any signs that either front tire had spun even though the back did.
Sound like it's not working?