Mix the dry rub ingredients in a small bowl. Pat the pork dry and rub the spices in thoroughly, working into every crevice. Place the pork in the center of the slow cooker.
Roll the oranges firmly on the counter under your palm to soften them. Halve them, squeeze the juice into the slow cooker around the pork, and add the squeezed halves.
Separate the onion quarters into layers and scatter them around the pork. Add the garlic cloves.
Halve the jalapeños, remove the stems and seeds, and lay them on top of the pork. Pour the broth around the pork.
Cook on high for 6 hours or on low for 10 hours, until the pork shreds easily.
Remove the pork and discard the orange halves. Strain the cooking liquid, keeping the onion, garlic, and jalapeños. Skim the fat from the liquid and set it aside for crisping.
Simmer the defatted liquid in a small saucepan with salt until it reduces to about 1 cup.
Blend the reserved onion, garlic, and jalapeños with half of the reduced liquid until smooth.
Shred the pork into bite-size pieces and toss it with the purée. Add more of the reduced liquid if more moisture is needed.