@Malcolm_Tucker I figured it out. It's because the CenterOfMassOffset is set too low(too far downward/negative along the Y axis). You can fix it by making the middle value(Y axis) in the CenterOfMassOffset line more positive. Which makes complete sense if you can envision in your mind's eye how the vehicle would act if the majority of its weight was centered below the height of the axles(which is how it's been set in this instance). The reason vehicles behave the way they do IRL is because that's not the case.
For example:
<PhysicsModel Mesh="trucks/bmb6l9q">
<Body
CenterOfMassOffset="(0.1; -1.400; 0)"
ImpactType="Truck"
Mass="3230"
/>
</PhysicsModel>
Change to:
<PhysicsModel Mesh="trucks/bmb6l9q">
<Body
CenterOfMassOffset="(0.1; -0.6; 0)"
ImpactType="Truck"
Mass="3230"
/>
</PhysicsModel>
I used the -0.6 value from the other 98 Discovery mod which behaves correctly. I don't know exactly how much, or how little, adjustment actually needs to be made to totally fix it. But I tested it with that value, and it seems to work much better.