In the savages, there's only 2 spider gears in the diffs. I've added 2 more to mine to strengthen it up. My diffs are holding up fine to the bigger motor. Not sure how the t-maxxes are setup, but I'm sure it's the same concept.

I know of aluminum diff cups, but they are a tad expensive for the savages. I think they are around $50-$60 each. I'd rather just replace it with a stock cup if it breaks.