Question on MiniAussie with weak back legs (tucson)

We have an almost 4 YO mini Aussie that is suffering from loss of her back legs.
We've been to all the vets/neuros and no one has a clue as to what this is, or what to do.
It is not any type of dysplasia.
She was purchased from a Pet Store on Wilmot just North of 22nd St that has since gone out of business.
We did contact the breeder, and they had no idea.
.
.
and we are not looking for fault.
We're looking for anyone with a Mini Aussie possibly from that same litter to see if they are experiencing the same issues.
Thank you.
Put AUSSIE in tag line.
Please do not respond with pets to rehome or other services.