Those vehicles are step above your typical road truck. They are often more prepared for the harsh terrain and may be tuned even further to give a proper off-road a hard beating. However they are often underperforming in a stock form and may not be too productive without proper upgrading. Some of them are capable of rare custom engines that have to be discovered first to be installed in a vehicle.
While heavy duty trucks may not be as accessible and reliable as proper off-road vehicles, they are often cheaper and easier to obtain.