I assume your differential looks something like this:
[ATTACH=CONFIG]84457[/ATTACH]

The inside ends of your axles snap into the gears labelled #4.
The shaft (#9) & pinion gears (#10) connect the two axles and put power to the wheels.

One other thing to check on each axle:
-hold the shaft of the the axle in one hand
-try to turn the inside end of the shaft with the other hand
(maybe, one of your axles tripod bearing popped off some how.)