Could a slipped disc in the upper back, near the shoulder blade, be caused by a shoulder dislocation?

Yes. This would depend on the cause of your dislocation. If it was traumatic, it is very likely that you could injury a thoracic disk. Also, a dislocation caused by a seizure could have enough muscle contraction to lead to a herniated disk. In addition, a select group of patients experience shoulder blade pain due to an inflamed bursa.