Like they said above, spin the back wheels by hand, and see if the driveshafts move at the same time. As for the hexes and pins, when you take the wheel off, you will see a plastic, 6 sided piece that goes inside the wheel. That is the hex. Once you take the hex off, you will see a metal pin (be careful not to lose it), that goes inside of the back of the hex. Thats what makes the wheel spin when the driveshaft spins. If the hex strips, gets rounded off, or the pin breaks, or the hex on the inside of the wheel breaks, the wheels wont move. Next, hook up a battery, set the car on something off the ground, and, while holding the throttle on the controller, see if the rear driveshafts turn when the front ones do. it could actually be several different things that could be wrong. The hex could have rounded off, the hex pin could have broke, the diff could have broke, the shaft between the transmission and rear diff could have snapped. Best bet is to get the manual, so you can see how things are connected, as well as how they come apart.