Given the 3 weeks since this occurred, you are past the wait and watch phase.
I would recommend getting a diagnosis. If there is an orthopedic specialist or a vet with orthopedic exam and treatment skills in your area, start there. You should be able to ask your vet and get a referral. I'm suggesting this because you need someone who has experience with orthopedic injuries and trained hands to do the physical exam part of the diagnosis and determine what needs to be x'rayed. I'm suggesting going through your regular vet because it keeps him/her in the loop and is a learning opportunity.
Depending on the findings of the exam, x'rays of the hind leg/stifle and pelvis are likely to be needed.
Soft tissue injuries require rest to heal. Stifle anterior cruciate ruptures do best with surgery. It doesn't sound like a long bone fracture - both the vet and chiropractor would have found it, but hips and stifles can have other bone/ligament damage.
Keep him quiet or as quiet as possible. I'd give rimadyl and not tramadol, but have only my personal preference for that statement. I think that orthopedic injuries are inflammatory and rimadyl treats inflammation. Tramadol treats only pain.
I would very much appreciate knowing the outcome if possible.
I wish you and your crazzy dog the best. I have a bunch of crazzy Dobes and love to see then in action.
Let me know if you have further follow up questions.