You're going to need to look at your truck, and see if there are any hoses that are near that location on your truck. I can't tell by looking at the picture if that leaf spring mounting point is in front of the axle, or behind it, towards the rear of the truck.
Regardless, the chances of that being axle fluid coming from that location is remote. How would it even get there?
If there's a brake line that runs near that portion of the bed/frame, then that is a possibility, but you're going to need to climb under there, and look around to find the origin of the fluid.
Is there anything in the bed of the truck?
BC.