I would suggest a three tiered approach:
1. Get a referral from your GP for an Orthopaedist
2. If possible, do a consultation with the surgeon who did the original knee replacement, sometimes the elements get loose
3. Go to a Chiropractor for the back pain. Even if your insurance covers it, Chiropractors are Portal of Entry physicians, which means that you can walk into the office without needing an external referral