Wouldn't recommend the RC4WD R2 tranny. I have it in the worminator 6X6 and it is VERY slow. Big gear reduction, so lots of torque but no speed. No need to explain anything to us Gary, most important is you take your time and enjoy the build as well as learning some stuff along the way. Have you thought going gearhead motors directly to the sprocket and avoiding the whole idea of a diff. Most of the r/c construction stuff goes this route and their are alot of options out there and you could then use a sabertooth ESC for track control. Now your probably gonna think, hey this is all for 1/14 stuff and yea your right but these machines are between 50-100lbs easy. My excavator weights in at 42 lbs and is on the average size.
https://www.servocity.com/motors-act...ry-gear-motors
http://shop.premacon.com/product_inf...roducts_id=697
http://www.cti-modellbau.de/Baumasch...9-210-214.html
http://www.cti-modellbau.de/Baumasch...8-241-260.html
http://www.cti-modellbau.de/Ersatzteile/
http://www.rcbrmin.com/index.php?mai...roducts_id=456