The Tamiya TT-01 rear gearbox loose problem is a common issue among the TT-01 chassis. This is when the rear gearbox housing aka differential case becomes loose, which can result in drivetrain instability, gear misalignment, or even stripped gears over time.
The screws securing the gearbox halves may loosen over time or strip the plastic threads, or that the plastic gearbox cases may deform due to crashes, heat, or over-tightening. The rear shock mount which is part of the rear case is particularly vulnerable – if the sock mount is broken, the connection to the chassis is loose, and the rear diff case will come off pretty easily.
A 3D printed solution can be found here: https://upgradeparts.com/2025/06/11/tamiya-tt01-rear-gearbox-case-fastener/