I have a crouched trembling gait due to pelvic fracture. What brace would reduce knee flexion?

No brace but exercis. If you can you need to do quadriceps exercises to both strengthen your thigh muscles to support you when you can stand and walk fractures take 6 weeks to heal. Be certain to do range of motion to maintain motion in the hips and ankles and feet as well as your knees. This will avoid contractures.