Tom
it's more likely that the diff was either:-
- slipping too much
- not built correctly
- already worn out
running your gearbox "backwards" won't make a scrap of difference. If it did, the gears would be marked as "directional", i.e only to rotate in one direction, and they are not.
I had a diff in my MF2 do that, and it was because the spring in the diff half was knackered, and i overtightened it. The added friction melted the diff and it ended up in two pieces, probably similar to yours!