prop the truck up on a table so your tires are free to spin. now spin the spur gear, does that spin both rear tires? Both should turn in same direction. Now hold one tire and spin the spur again, only 1 will spin, then try the other side. If only 1 turns, then your pin is missing on that side, if both are fine and everything spins it could be a few things, slipper not tight enough or the cluch shoes arnt grabbn. If neither tire spins, you could be looking a stripped gears, run those few things down b4 rippn it apart, could be just a bad clutch.