Those #s don't seem like a shunt to me in terms of a single portosystemic shunt as the postprandial readings usually are over 100 in a single shunt. Your dog may have microscopic shunting known as Hepatic Microvascular Dysplasia or "MVD" which is very common in yorkies.
Your vet has given some great advice to see an internal medicine doctor! You should consider making an appointment and letting the internist evaluate AND examine your dog. They will likely recommend a diet change to moderate protein. Many dogs with MVD and even shunt eat Hill's prescription L/D diet. Our dog was on L/D for a while, but she has many other medical problems so did not do well long term on the Hill's. We now home cook for her and her numbers have gone over 100 to normal and she is age 10
Good luck - you'll get to the bottom of this soon.