I can't think of anything anyone could do, intentionally or accidentally, to damage a transmission to cause such a problem. First of all, you listed a van with an axle in the transmission. Are we talking about a front-wheel-drive Caravan / Town and Country minivan or a rear-wheel-drive full size van? What is "okay", the transmission, the fact the axle is falling out, or the axle itself? Were four replacement axles installed and they all came out of the transmission or was the original axle simply reinstalled four times? What happens when this axle comes out?
The most logical conclusion is this a front-wheel-drive minivan, the original axle is being reinstalled, and there's a broken spring inside the inner cv joint. Those springs used to cost around $3.00 from the dealer. Is there any other related history such as the vehicle was crashed or engine mounts were replaced? Any abnormal vibrations or shimmying?