Hey Bondsy
I'm pretty sure the caps shouldn't spin. It probably means the yoke ear is stretched. You'll probably need to replace the inner and outer axles. Good excuse to pickup some chromoly axles or upgrade to RCVs.
Tony
what tony said. not really a big deal if you have full circle clips but, if you're still running factory shafts with c-clips, this will lead to a failure. if you are still running factory shafts, you can tack weld the cap in place and that will help until you can't afford to upgrade to chromoly shafts with full circle clips.