Prednisone starting 40mg/day taper over 8 mo. Sle. Hip/knee pain severe past 2 months. Hip pain totally new. Pain chronic. Possible avascular necrosis?

Possibly. Hip pain from avn can be felt in the groin, thigh or knee. It may hurt at night or with activities. X-rays may or may not show evidence of the condition, while an MRI scan is very helpful in detecting the condition and extent of involvement of the bone. Treatment may include core decompression or hip replacement depending on extent of involvement.
See rheumatologist. What you describe may have several causes. Avascular necrosis is definitely in the differential diagnosis. You are on "heavy duty" medication with Mycophenolate Mofetil (cellcept) and prednisone. Don't wait, go and see your rheumatologist.