Hello,
As many have discussed on Discord, we came to the conclusion that the Mack Defense M917 has a too low collision mesh, making it drag its axles too much on the ground and obstacle making it hard to drive.
As per the model, the mesh ends just below the axles, but other trucks have their mesh end up about in the middle of the axle.
The video below shows a good summary of the situation.
@nakedDave on discord made a mod which modify the collision mesh and I must say, it now performs much better and becomes much more enjoyable to drive.
May I add that the Derry Special had a similar problem when it first came out but its mesh was modified, and it now is a very fun truck to drive.
For an example, here I am, with my back axles grounded on some roots, and I cannot move any further without using the winch.