I agree with others that this is not good. To be clear though, the play is in the pinion, not the drive shaft, so I don't think you need a new drive shaft or to get your drive shaft repaired. You do need to fix something in your pinion/differential though. It could be that you need to just tighten the nut a little, but It's tough to say whether or not something inside the differential is bad. I'd start by removing the diff cover (over bucket or oil catch basin). Look for bits of broken metal in the oil, little to no oil, or visibly worn/broken parts. That might give you an idea as to what needs to be addressed.