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If he is dragging his back legs AND having trouble walking across the floor, I would be very concerned that he has a back or disc injury that is causing pain and/or paralysis.
You need to get in contact with the owner and get permission to take the dog to the vet right away.
There are some great prescription anti-inflammatories that can help with this.
Let me know if you still have questions.
MRI's are certainly a possibility but most regular vets don't offer this but they can refer you to a specialist that does.
Check out the link above for more information about a common disorder in shepherds.