http://www.defiance.com/en/series/ph...ance-episode-6
"This is the first episode we get to see the lawkeeper charger in action. The engine's been replaced with a Votan-hybrid power plant that runs on petrohol (a byproduct of gulanite that's naturally progressed through Hellbug shtako). The shocks have been fitted for navigation across the rugged terraformed landscape, and much of the body work has been armored against attacks by Raiders and other predators - natural or otherwise."
I imagine that a similar deal has been done with our Challengers (including adding 4WD), which also explains why the engines sound weird.