2 bugs with this otherwise beautiful truck:
diff lock does not actually lock, some wheels dont even turn while others are spinning
the low gears (plus, middle, minus) seem to alter power output not gear ratio, this may work well.enough for other trucks but in such a heavy and low powered truck the lowest gear needs to be an increase in torque for getting up hills, not just lower speed.
If diff lock is programmed by powering each wheel separately instead of actually monitoring wheel speeds and adjusting, then perhaps increase power to the wheels when in diff lock but limit the maximum speed so as not to give an unrealistic boost, this might work for the second issue as well
Also, a flat deck would be nice on the heavy trucks in real life it is very easy to fit a flat deck to absolutely any truck frame, but this is just a suggestion.