I'm not sure how the heater core hoses are routed from the thermostat and
the water pump to the firewall, but I would take a good look at both lines
for holes/cracks. Somewhere under the engine bay, they probably are secured
to (a) bracket(s) that runs from the front to the rear of the engine, and
then to the firewall, which would make it easy for them to drip onto the
transmission if they were in fact damaged. Pressure test is the best way to
test this. Or if you can pinpoint the drip from underneath the truck and
place a drainpan under it, start the truck (make sure the antifreeze level
is OK) let the system pressurize for a minute or two (not very long), shut
the engine off and carefully search for your leak. Remember that this
should be done with the engine cold, you don't want to deal with hot
antifreeze spraying you in the face when you suddenly "find" the leak.
Honestly, since you don't know how large this leak is, it is probably a
better idea to pressurize the system with the engine off. It may be time
for a rad. shop to test it for you.
If it is the heater core lines, replace both and not just one. I would also
take a real good look at the radiator hoses, as it is probably time for
their replacement as well, even though they may be OK now. Preventive
maintenance can save you big time down the road, believe me.