Mark,
What the consciences are, along with the impact, is akin to hitting the center section of the spinner, causing an abrupt downward motion. This movement, along with the weight of the other suspension parts, tends to create a downward pull, which in turn causes the stub axle to want to shift out wards, causing the c-clip to pop off the end of the splined section inside the diff.
Sorry I can't justify the inertia movement any better, but believe me Mark, this is what the cause was.
Like yourself, the first time this happened to me I thought it couldn't have been caused by the downward force that was being asserted with the hammer, but guess what a few years later the exact same scenario produced the exact same results.
Then the fun begins since you have to remove the complete spare carrier, remove the half shaft, disconnect the rear spring, drop the rear x-member, spill out all the differential oil over yourself, before getting to the rear cover . Then you have to get inside, remove the clip, and reverse the disassembly process.
All this for one fûcking hammer blow, is in my opinion a very high price to have top pay. As for the shïtty little c-clip, you have best chuck it and replace it with a new one. For those of you in the know, the c-clips in their ( GM ) next generation pumpkins are almost twice the thickness of these, and require expensive, professional, tools to be able to remove, or install them.
Have a good one buddy,,,
regards
Stepinwolf