Some end play is acceptable, not a lot if more than a sixteenth of an inch or so, it could be reason for concern. Nothing a quick inspection wouldn't solve.
First look at the axle seals, if they are not leaking, i probably wouldn't go any further.
If there are signs of wear or leaking, pull the diff cover off and take a look, you er going to have to take it off anyhow to replace the seals. If you replace the seals be care to protect the ABS sensor.
Once you have the cover off one of the areas you are going to want to look at is the pinion shaft (pin) it's what keeps/limits the axles from sliding in or out keeping the "C" clips in place, preventing the axle from falling out.