Thanks for writing and sorry for the delay in response.
It sounds like the pain is from the SI joint area.
Normally, during walking, standing, or running, weight (a vertical force) is transmitted up through the leg bones, through the SI joint and into the spine. What keep us upright and supported is the rigidity of the spine, pelvis, SI joint, and leg bone with their respective ligaments and connective tissue and the muscles surrounding these bones.
When any of these structures are damaged, then normal function (walking, standing, etc) can cause pain.
Since the SI joint is a weight bearing joint which translates a great deal of force, it is important to support the area. This is why our SI area was created with thick, bulking, strong muscles (the gluteals and iliopsoas) and dense and stiff ligaments.
Besides the treatment measures already mentioned, I would focus effort on aggressive strengthening of the muscles which support the SI joint. This will include the gluteus muscles and iliopsoas. Also strengthen all the hip and trunk muscles (abdominals and lumbar muscles). This way not only deal with the SI area but condition the areas above and below the pelvis.
Give yourself 9-12 months to do this. See a physical therapist if needed.
Starting off, aquatic exercises may be best as buoyancy takes weight off the legs and therefore the SI joint.
Land exercise should include deep squats, lunges, and a special exercise for the SI (With both hands, grasp an overhead bar so the feet are dangling off the ground. Bend both knees simulaneously up toward the chest. It is like a reverse sit up. This will strengthen iliopsoas, abdominals and latisimus dorsi. Use a step stool if needed and definitely do not hop down - gentle step down.). Again, the idea is to strengthen the muscles around the SI joint.
Take your time and strengthen gradually.
Keep us posted.
ps I do not know of any SI specialist in the Northwest. See an orthopedic physical therapist may be the best bet.
Tom Plamondon PA-C, PT