TWO SPARE brushless diffs? Well, if you have a couple new chassis, I guess you didn't over-buy. You'll never strip the diff housing teeth, but the planetary gears inside will wear. And you can buy just the planetary gear set cheaper than the whole diff. That TrakPower gear grease will really help.
Now that you have a metal diff gear, you NEED to use blue loktite on all eight diff screws, otherwise they will come loose, the internal gears will loose mesh, and strip out almost instantly. I would suggest taking it apart and cleaning out the screw holes, so you know that the loktite will work.
1/8th scale RCs are supposed to be tougher than 1/12th scale stuff. You weren't running that 4S 2100kv with 6in tires on GRASS were you? That's some serious power, but 6in tires can put a tremendous strain on axles with that kind of twist.