Ok, so the truck is a 4x4. Is there any oil/dampness around the rear transmission mount area (where the transmission mates to the transfer case), or the modulator valve (little pod looking thing on pass rear of trans with vacuum line going to it).
Also do you know what kind of fluid the transfer case is serviced with? I have seen some serviced with gear oil, and I have seen some serviced with ATF. Check the level of the transfer case for an "overfilled" condition (pouring out of upper plug hole). If the rear seal in the transmission goes bad, I have seen them fill the transfer case up enough to leak.
Hope this helps enough to get you started chasing the leak. If that turns up with no results, then try the cardboard trick to narrow down the location the fluid is coming from.