I'll bet it's just water condensing on your AC line and dripping down.
I noticed the same thing last week when we drove from Glacier NP up into Canada on a very hot, humid day running the AC.
As long as it's just water it's nothing to worry about. It's not even coming from your truck, it's just that the AC lines get very cold and then water condenses on them and when you stop it drips down.
If it was oil, antifreeze, transmission fluid or even washer fluid it would have a distinctive smell or color.