even with powerful Nitro Engine hellfire difs have problem!
Try Hot Bodies truggy Crown and pinion gear
http://www.amainhobbies.com/product_...oducts_id/2898
http://www.amainhobbies.com/product_...oducts_id/2897
Also diff housing is too week under the load it expands and you strip gears, why thats spiral cut gears works well in hellfire.