Traxxas Pro-BUILT differentials come professionally assembled for easy drop-in replacement. The sealed assemblies come pre-filled with 30k-weight silicone oil with steel gears to withstand Velineon® brushless horsepower. Pro-BUILT differentials are direct replacements for VXL 4X4 models (see full list below). Note: Pro-BUILT differentials are not directly compatible with Titan®-powered 4X4 models.