Yes, thanks for reminding me I too run ball diffs in my V2. I couldn't believe Tamiya did not include this in the V2 package.
That would have been a nice addition!
I have found the more openings in the chassis, also allow the ingress of small pebbles and dirt, which we cause the middle gear to strip quickly. I have found blutack and battery tape my best friend to combat this.
I run a strip of battery tape along the bottom of my chassis to seal up that seem along the bottom of the chassis. This has prevented any/all debris from getting in my chassis and has acted as a protective strip along the bottom of the chassis. I have been racing my V2 w/ this tape and have not run into any cracks in the chassis or had any debris in the chassis and gearbox. ...The tape may help solve your chassis cracking?