well...it's also not a leak inside the truck either...it's just a SLOW leak...
you WILL need to have a look at your transmission...look for any sort of fluid residue. Check near the front of the trans. where it connects to the engine, if it's auto check around the torque converter, check at the back of the transmission around the drive shaft. check and make sure the drain and fill plugs are tight...
inspect the transmission for the leak then check back