By looking at your case file, we can see that you increased your Vehicle Trajectory integration timestep from the default of 0.025 to 0.1 seconds.  With such a large time between calculations, the axles on the vehicle have become dynamically unstable.  By setting your timestep back down to the default, 0.025 seconds, your vehicle will behave as expected.  If you are going to be looking at a collision using DyMESH in this event, we recommend that you further reduce the timestep to 0.001 seconds (see DyMESH Timesteps in the DyMESH Options dialog).