Kevin, the two pictures you provided are somewhat low resolution, so it's difficult to actually see the culprit. Typically I would have guessed that one (or more) of the plastic gears has a crack in/on it and is spinning freely on its shaft. This is super-easy to check; just stick your finger on the broken brown gear (the one that is almost black with old grease and has angled teeth on it) and then try to roll the truck across your table top while keeping the brown gear from moving.
If it rolls easy, you should be able to see which gear (or gears) is (are) the offender(s).
If it doesn't roll easily and attempts to shred the flesh from your fingertip, then, a) quickly stop pushing, and b) take a closer look at that brown gear, specifically on the left edge of the gear teeth. I tried to blow your pic up to get a better look, but it's not a hi-res picture. That said, it APPEARS that the left edge of the gear (as viewed in your picture) is a wee bit shredded. If that is indeed the case then stick your eyeball on things and see if the worm on the armature shaft was not hitting the brown gear fully in the center - either because the brown worm gear was mounted poorly from the factory, or because it moved over time. Try to slide the brown gear left or right on the shaft. If it can be moved with just your finger, there's a slight possibility to glue things back together using the existing parts and very little work on your part. But first tell me if what I just described is the case.
Replacing the brown worm gear with a new one is not a simple task, but certainly can be done. But let's see what problems you find first, then you can decide how much you are willing to attempt yourself or if you would even want to spend some bucks to have someone else fix it.