Although it seems strange, this is not a problem. It is what a differential is supposed to do. It is kinda hard to explain, but if you popped your rear cover it would all become clear. The axles are designed to be able to turn at different speeds through the use of planetory gears. The reason for this is basically geometry, in going around a corner, the outside wheels have to travel farther then the inside wheels. (Think about the last time you saw a (non-posi) vehicle stuck in snow or mud, one wheel can be spinning 40mph and the other not turning at all.)
Steve O
