the Yokomo ball bmax 2differential is really good but follow the tips on the link for sanding and greasing the differential will make very smooth and last a long time.
http://www.teamirsrc.com/techtips.html
I do have the gear diffrential for the yokomo but you still have to maintain it and replace the gaskets and o-rings and replace the oil when it goes black. I also think you have to bed the gear mesh in as well as the gears are die cast and don't have a nice smooth hardened surface that you would get with a machined gear.