A small amount of movement and clunk is normal. How much does it move? There are several parts that can wear and cause too much. The axle gear, the carrier, the clip, the spider shaft and the end of the axle itself. The only way to know for sure is to pull the cover off the diff and have a look. You will need to pull the pin and spider shaft out, push the axle in and remove the c clip. Pull the axle out and then remove the axle gear to inspect the back side of the gear and the carrier. I have seen the gear and carrier wear to the point the clip falls off and lets the axle come out going down the highway. The rear has usually been run low on lube when that happens or really had the heck run out of it.