Think of several precision-ground longitudinal half-round grooves in both the inner and outer (or male and female) axle parts. Line these inner and outer grooves up to create a series of round holes. Fill each hole part way up with a bunch of bearing balls. Axle plunge causes the balls to roll in the grooves even while transmitting tremendous torque. This setup is very strong and durable. If you size them right, keep the dirt and water out and the lubrication in, the lifespan can be nearly unlimited. These have been around for industrial use for many decades and in limited automotive use since at least the sixties. A very close cousin to these can be found in most common worm-and-sector steering boxes (Saginaw, etc.) and ball screws in milling machines.