rear diff

Hi all Had a slight noise was a strange noise about evey 2 seconds mainly noticed when i came off the gas, but could be heard slightly over noise of truck. seemed to be coming from diff but wasnt fully sure, so i removed half shafts and noise stopped so decided it was the diff.Took diff out and have had a good look no metal in oil no broken teeth, cross shaft is intact, all gears are nice and shiney. but i have slight play one one side of what i think are called the differential side gears the other side are rock solid... so guess what i want to know is is any a mount of play a bad thing is a small amount ok? is it time to change the diff, or do i slap it back in..

Thanx Dean