CT scan shows Degeneration of the sacroiliac, could a nerve be pinched there, causing my urinary retention? Would an MRI show if a nerve was pinched?

Perhaps. Nerve compression can sometimes be s cause for retention. However usually you will have other symptoms also related to nerve compression such as pain, numbness etc. Do you have any other health issues such as diabetes? This can cause bladder weakness also as a cause of retention. An MRI could diagnose if you have nerve entrapment.