Diagnosed my brothers dakota like this. Put the both axles on jack stands get both front and rear wheels well off the ground. Start it engage four wheel drive put it in gear and let off the brake. Don't gas it much chain transfer cases don't like all of the slop and without the friction of the ground things like to clunk and bang.
So with your foot off the brake and your buddy watching teh front driveshaft does it spin? If so and you don't have manual lockouts your left with two things. The front ring and pinion or spider gears like you mentioned. If the front driveshaft and the front axle shafts are spinning but not the tires then maybe the auto hubs.
If the front driveshaft isn't spinning its your tcase chain or planetary gear set.
My brothers dakota smoked the front spiders once and the tcase planetary once.