If yoy have the front only off the ground with the 4x4 engaged you will not be able to spin the drive shaft because the front and rear drives are connected, if you spin one wheel the other wheel should spin becaus of the spider gears and the drive shaft is locked.

If you dissengage your 4x4 and turn your front drive shaft by hand the half shafts should rotate.

It is hard for me to believe that the diff is blown to the point that the pinnion is spinning free and not causing all sorts of noise and banging.

It is likely that your front vaccume hubs are not working properly.