Hello I found and upgrade in the second Yukon map (level_us_04_02), the upgrade said that is the special offroad gearbox for Freightliner M916A1 and Voron 53233, but this is the box both trucks has already.
Keep it improving the game, Greetings
I imagine it's a placeholder and will be later replaced with an actual upgrade. Would be nice to have Highrange gearbox for those trucks or some custom gearbox like F-750.
or a decent motor for the apache