What are you describing as the "outer axle"? Is the CV axle pulling out of the inner CV joint?
There are two types of inner CV in use, the Glaenzer (commonly known as a tripod or tulip joint), and the more common Rzeppa, with 6 steel balls - OEM axles use the Rzeppa for both inner & outer joints, most after market have a Rzeppa for the outer CV, and a Glaenzer for the inner CV - the inner joints on the OEM axles have a retaining ring to stop them pulling apart, the after market axles don't use them - this may be your problem.
I'd be tempted to machine a circular "cap ring" and tack weld it to the outer end of the tulip on the aftermarket shaft - where it would serve two purposes - the first being to prevent the axle from separating and the second to strengthen the tulip.