Rear axle drive wheel

I noticed today while working on my 67 ambo that the left wheel is the drive wheel.  I thought the drive wheels on an open were always on the right?  Also while the left wheel is moving quickly the right wheel moves a bit, very slowly.  How is that possible on an open rear?

There is no "Drive wheel" if you look at any differential logically!
As Trader says - which ever wheel has the traction will move the vehicle. There's no drive wheel. Each receives EXACTLY the same amount of torque. They have to - the carrier gears are both pushing on the axle or side gears equally. 
Now under really heavy acceleration there is a tendency of one side to raise up and the other be pushed down due to the forces counter-acting the forces on the pinion. And the pinion wants to climb up the ring gear. But then again, we're still talking that both axles receive the same amount of torque - if one lets loose it's because of traction, pressures, those forces, not that one is a drive wheel and the other not. It's about weight transfer. 
All things being equal, if the brakes were removed, bearings had the exact same resistance to spin, with the rear off the ground both wheels would spin exactly the same.