if the seal is leaking you will see the fluid dripping at the seal, it will also be sprayed on the floor of the truck above the seal from the spinning driveshaft. if it's dripping at the u-joint the plug in the end of the yoke is leaking(behind the u-joint) this will throw fluid above the u-joint on the truck floor. I've seen many plugs leak like this. remove the driveshaft, remove the front u-joint, pop the plug out from the yoke, clean it well, as well as the groove in the yoke where it seals put it back in with some permatex sealer and put everything back together, probably a good time to replace that u-joint. the fluid travels down inside the yoke through the splines and seeps out of that plug. if you don't fix it right away be sure to fill your transfer case regularly.