with a properly spec'd tandem axle the weight should be balanced so that AWD will not be needed. Honestly i would look at a good used redi-mix chassis. spec'ed for the DIRT and off road situations not to much power to brake driveline or get drivers in trouble. a good wheel base and frame length for a 4000 gallon tank. personally i think you are wasting your money with a stainless tank. you can buy 2 epoxy coated steel tanks for the price of ONE stainless tank.