Well, I certainly hope the mylogram shows good news tomorrow.
My friend's dog had what sounds like a very similar issue about a year and a half ago. They never found out what caused it (out hiking in the woods, and the dog was out of sight), but she came out of the woods crying and unable to move her tail. The e-vet misdiagnosed her, and just sent her home to rest. The dog refused to go up or down stairs, wouldn't go outside, and rapidly lost muscle in her hindend. Tango sounds like she has a similar personality to Penny - Penny lives to run in the woods, and my friend had told me before that she had made the dog a promise: she would never make her live a life where she couldn't do what she loved.
In a last ditch effort, they took her to an ortho specialist, who quickly diagnosed a slipped disk and did a mylogram to confirm. Penny had her surgery, recovered faster than anyone thought possible, and is back to running in the woods every day with zero pain.
Here's hoping that Tango has the same success. Good luck to you both.