Did you remove the plastic HVAC plenum screen and check the hose connections and heater control valve? Leaks in this area will usually drip through the plenum drain and land on the transmission.
Carefully check around the auxillary coolant pump. Sooner or later they will begin to seep.
Also, don't overlook the coolant pump and hose connections up front. Small leaks often migrate along the engine and drip at the back even though the source is up front.