I only have tc3s, but I think they shared drive lines with the tc4. There are steel out drives that came with the nitro TC3, but they're pretty thick and heavy. The diff gears were still the yellow-ish composite on the nitro. NOS parts seem to still be available, or lots of used parts out there to keep it running.

I imagine new diff balls are easy enough to find since they should be interchangeable with new stuff.