Well I’m not an expert on New Process Gear units, but…
If the full time mode does use a viscous coupling, it’s rare for them to break. They are usually extremely reliable. They work by shearing a high viscosity fluid causing the fluid to expand and putting pressure on the coupling causing it to lock the front and rear shafts. By just putting the truck on jackstands and letting the wheels spin (I’m assuming for safety sake you are just doing this off idle) probably will not be enough to shear the fluid enough to lock up the coupler.
If it uses an open center differntial for full time mode you could see this same scenerio on the jackstands. You have a very low amount of drag on the system and thus power is going to go to wheels with least amount of resistance. Which is typically the rears. If your center diff is broken odds are it lost some parts, ie: broke some gears. You probably would have noticed this unless they are sitting safely at the bottom of the tcase.
Part time would work fine provided nothing is fucked inside. As part time physically locks the front and rear driveshafts together by way of gears.