Hi everyone, I’m hoping to get some help regarding an issue with my 2015 C300 4MATIC Sedan. My car uses the 7G-Tronic transmission, and I recently replaced the front differential with one I bought from a part-out seller (no VIN from the donor vehicle was provided). After installation, my OBD scanner shows the error: "Front differential not OK."

At first, I thought all W205 front differentials from 2015–2018 were compatible, but I’ve since realized there may be key differences between models with 7G-Tronic and 9G-Tronic transmissions. I now suspect the replacement part may have come from a 2017+ model with a 9G-Tronic, and this may be causing the issue.

Because my transmission already has some issues, I originally brought the car in to diagnose a different problem: when shifting into Drive or Reverse, the whole car jerks violently—as if someone is pushing you from behind. Also, when accelerating, the car shifts into higher gears normally (e.g., 4th), but after slowing to a stop, it doesn’t downshift. It stays stuck in that higher gear and tries to take off from there, which is clearly not right.

I’d really appreciate help with the following:

• What is the correct OEM front differential part number for my specific model and transmission?
  
• What steps should I take to fix this incompatibility?
  
• Is there a risk of damage to my transfer case or transmission if I continue driving like this?
  I have attached all the photos I have for the part out front differential.
  Any guidance or advice would be incredibly helpful. This is my daily driver, so I’m hoping to resolve the issue ASAP. Thanks in advance!