actually my car used to run a combo of solid engine mounts and rubber gearbox mount. it was the incorrect uni
joint phasing that was the cause of the broken gearboxs. im now running solid engine and gearbox mounts and its all good
because the driveline angles are correct. so solid mounts are not to blame, although i dont see a need to ever run them in
a road going car.

lifting the gearbox higher will change your driveline angles slightly and most likely cause premature wear of your
uni joints and maybe some driveline vibrations, but i wouldnt expect your diff to fail instantly because of this.

if i were you id be going back to a factory rubber gearbox mount which will transfer less vibrations through the car
and most importantly return the driveline back to its correct position. it may even make your suspected broken diff symptoms go away