[Espeefan]

There are a lot more options these days then in the past, if you are okay with less then 3 gears. The best way to decide what will work the best is to consider the type of truck you want to build, and then consider what you want to spend. If you are tight on space, an R2 would work well. Axial transmissions from the rock crawler trucks work nice too. Or search for a compact planetary gearbox, or gearmotor. There are also the Tamiya 3 speeds, Wedico 3 speeds, Scale Art 3 speeds, and some have used the Traxxas E-Maxx transmissions as well. Just about any of these would work well and be pretty much bullet proof.