Well, i splurged on some new ETO gearboxes for the Sherman and I can't tell you how much a of a vast improvement they are over anything out there. If you get the chance to buy a set from ETO, i highly recommend them!
I ran these gearboxes at Danville during the May event and they performed flawlessly. they chewed up the terrain with the metal tracks, never faltered and I think they are indestructable. Because I am running Mato drive sprockets i had to have new out shafts made by Daryl Turner, they come set up for tamiya drive sprockets.
if you want some good gearboxes, you would not go wrong with these!