As it was already mentioned by multiple reviewers, the rear axle of the Zetros seems to have a potential problem. Under constant heavy load pushing the truck beyond its limits, a 5L beam that holds the gear driving the rear differential gets pushed out a little bit, and as a result the gears start to slip, crack and might get damaged on a longer term. I tried to run several tests to see what might be the cause, can it be a friction variation of the axle pins or is it something else. We also tried to come up with a quick fix with @Attika that eliminates the issue under "normal" circumstances without completely rebuilding the rear axle. You can see the details in this video: