Inspection Dye for the A/C system...is this where you're heading?

 

I am with you. I don't see how this fluid / substance came from your truck. Especially, since you're saying it's located towards the rear.

Maybe, it was on the floor before you pulled over top of it?

 

I got it!!

Rear wiper fluid? The winter de-icer brew is yellow, isn't it?? What I win?!!

If it is the line to be possibly damaged allowing the fluid to leak on the ground

 

I was thinking leak dye as well, or dehydrated cat is peeing under his disco

 

Now it's yellow snow!
My windshield washer fluid is blue and has been for a long time.
Again it was after a long drive (5+hours). It's closer to the front than I originally thought. Wasn't using AC (obviously).
Total mystery to me.
Close up.
Dark patches are where the front tires were (with the rear being towards the right and front to the left).

 

The AC system is always on even if you are using the heat. Those stains look further forward than the AC drains though.

What coolant are you using? Looks like antifreeze to me. Throttle body heater plate is right above there.

 

I would do a pressure test on the coolant system. My disco pees all the time. It would behoove you to go buy a coolant pressure kit as you own a disco. If it is coolant and you have an ultra gauge you will notice slight temp increases as the days go on and you lose little by little coolant. Don't taste strange liquid, ahh lol..What if it was pee from a aids infected drug addict? I mean probably not but don't taste!! Smell it man lol Don't eat yellow snow it ain't lemon snow cones!