Is this what used to be called a broken backed hoof/pastern axis or otherwise known as sheared heels? My youngster suffered with this, it was due to incorrect foot balance due to rubbish shoeing and it made him lame. It was easily remedied over a period of good shoeing with a decent farrier. I am surprised it took xrays by your vet to determine this condition, the vet I used at the time spotted the problem immediately on sight when I summoned him due to the horse being lame. Still that's vets for you, if they can charge the cost of a call out to diagnose a problem or charge the cost of expensive xrays to diagnose the problem then I can guess which one they'd do, but then again I am slightly cynical!!
Before you shout at your farrier
I have to add its not always due to poor foot balance. It can also be due to conformational problems too which can create an uneven loading which will exacerbate the problem. Here is a good link for you to read http://www.equipodiatry.com/shearhls.htm
Very interesting about the treatment this chap recommends in the second paragraph under the heading of treatment
. Sounds like a very good idea and a lot of common sense too. Shame he lives too far away to visit/call him out.