After very similar troubleshooting and frustration. I did a runout check on the front rortors and found that my LF Hub had become warped. I dont know how or why. My only explanation for the vibration is. The rack and pinion stearing section is mounted to the frame directly below the engine mounts. The engine mounts are fluid filled for dampening. It seems any brake or wheel vibration caused a transfered oscillation into the fluid filled engine mounts and passed back threw the drive line. They work great for engine Vibs but not transference.
I bought an OE Hub from carquest assembeled and ready to install, bearings installed even had MB green grease in them. The vibration is gone smooth as glass especially at the 53-62 mph trouble speeds. This problem really didnt effect the car or gas mileage. I could never feel anything in the brake pedal or steering wheel. It drove me crazy. Finally got it, not to mention flex discs, brembo rotors, drive shaft support and a cv axle and new tires. Good luck hope this helps ya. I been down your path and it gets long.