RC4WD Gear Reduction

[9W Monighan]

Definitely sounds like a dry bushing. I don't like oilite bushings because they gum up.Especially in the dusty environment you work in. A good solid bronze bushing may work better. Better yet, can you install a ball bearing? Run the whole gear train in oil? I don't know anything about the drive you are using just going on what happens with dry bushings.
Another thing, Can the motor pinion be supported on the end? Probably not because the mounting holes are slotted to accommodate different pinions.
With the way you overload your trucks all the time, there could be some flexing on the pinion shaft making it run out of alignment with the driven gear.
Same goes for your rears that you said you have problems with.

[Lil Giants]

Originally Posted by 9W Monighan

Another thing, Can the motor pinion be supported on the end?

Yes it does, the motor pinion has enough stub sticking out that fits into a pilot hole of the GR output shaft, eliminating any motor shaft flex.