Stripped rear diff gear - solutions?

[The Doctor]

Hi guys, bought a lightly-used (standard) ZX5 recently and have so far raced it a couple of times. Really pleased with it, but tonight I stripped the rear diff gear. The grip was high (fresh grass, Ballistic greens) with some fairly large ramps, but I'm still a bit surprised. It was shimmed fairly tight but free running.

Is this a common problem with the ZX5?

Solutions I can think of:
1. loosen the slipper
2. full ali motor/input shaft mount
3. 3-racing ali input shaft collar for the rear gearbox.

Any advice?

Cheers,
Paul.