I agree with trying to lube up the u-joints as I did this last winter when mine started squeaking a bit. It worked!! But, I think you need to determine if his hubs are staying locked in when the dials are in the "auto" or unlocked position. This can be easily determined by ensuring that both dials are turned counterclockwise and reaching behind the wheel and turning the u-joints. If they turn you are free wheeling the hubs. If you is still getting this popping sound when the hubs are free wheeling then the problem is not the u-joints.