It is not a u-joint. The play is in the stub shaft to Unit bearings
Something is not installed right. I would check to make sure the nut on the end of the axle shaft did not back off. Is there a bearing on the inside to support that shaft like a traditional spindle? If so I would check that.
If you don't find either of those it is most likely a defective unit bearing. The unit bearing should be supporting the stub shaft and not allowing the stub shaft to move around like that.