Dragging hind legs

My understanding is that a disc problem will not normally show up on an x-ray.

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Intervertebral disk herniation is usually suspected based on the signalmen(breed, age, sex), history of appropriate clinical dysfunction, and a neurologic localization of the cervical or thoracolumbar areas. To make a positive diagnosis, spinal radiographs or Xrays will need to be taken. It is absolutely imperative that no movement occur and that the muscles along the spinal column be relaxed if an accurate assessment of vertebral relationships and disk spaces is to be made. Consequently, the only way to get good quality spinal radiographs is by administering a general anesthetic to the affected dog. The only exception would be if some other physical abnormality, such as severe heart disease, precludes this being done safely. In that event, surgical intervention is probably also not an option and specifically locating the offending disk is not necessary to carry out good medical therapy.
Once the dog is anesthetized, a series of plain radiographs will be taken with the dog lying on its side (lateral view). The veterinarian will be looking for changes in the normal relationships and density of the vertebrae and the disks. Those changes include narrowed or wedged disk spaces, displaced calcified disk material in the intervertebral disk space or intervertebral foramina, narrowed articular joint spaces, and/or many other suggestive signs. However, calcification of the nucleus pulposus does not necessarily mean that a disk will herniate. Unfortunately, most herniated disk material is not calcified enough to be seen on Xray. Therefore the exact position of the herniated material most often can not be determined on plain Xrays and a specialized study called a myelogram must be performed. Risks are involved and the possibility of worsening a delicately balanced situation exists. However, the manufacture of new less irritative dyes has removed a great deal of the risk and myelograms are now routinely performed without the great concern of the past.
In a myelogram, radioopaque dye is injected into the cerebrospinal fluid in the subarachnoid space. In a normal study, two lines of dye will be seen running on each side of the spinal cord more or less like railroad tracks running parallel to one another. When a disk has herniated, nuclear material and torn annular fibers can occupy the extradural space compressing the spinal cord. The normal parallel configuration of the two dye lines will be disrupted by either pushing them closer together or farther apart depending on the particular radiographic view examined. Additional oblique views can be taken to help identify which side of the spinal cord is more affected. By examining all views carefully, the amount of herniated material, and therefore the amount of pressure on the spinal cord, can be estimated. Also the preferred side to enter surgically can be determined.

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