The back is a good idea, might need a K9 chiropractor, your vet most likely would not be able to feel a misalignment.
However, that would most likely not explain the constipation. If the diet is right, that means that more water is being drawn into the urinary system and not enough into the digestive tract. (My dog has the opposite problem!)
My girl had a high WBC count and we used cipro on her, and it made a huge difference. (Turns out she had extra tissue in her vagina which causes ongoing problems that we had to remove.)
She also had inguinal hernias - lumpy areas in the groin areas, which are common in many of these small dogs it seems. They *can* cause blockage, so maybe something to look for, though I would think that the vet would have looked.
The crying is definitely communication to you that something is wrong, somewhere. You will have to be persistent to solve this mystery! I guess one thought may be whether the food is right, or if it's just causing too much bulk and dryness. Try adding some canned food maybe if you are feeding kibble?