Some dogs present differently...and not necessarily all or the same symptoms as another. Your vet is probably relying on the numbers of her BUN and creatin count to rule out liver shunt. These numbers are typically lower in dogs with LS.
Even if she doesn't have a shunt...it sounds like she definitely has some type of neurological problem. Has GME been ruled out?
http://www.yorkieangelpatrol.com/gmeinfosheet.doc