Answer:
The drive axles are the rear axles which have or can have power directed to them. On 6x4 truck (common to North America, southern Africa, Australia, etc), power only goes through one drive axle in normal operation - the one closest to the cab. A lot of tractors have a power divider lock which can engage the rear axle, thus, both of these would be drive axles. In instances where it does not have this feature, the rear axle technically would not be a drive axle, nor would lifting tag axles, etc.