With an open differential, on a yj, the axle on the passenger side that connects to the differential and runs about half way through the passenger side axle tube is called an intermediate shaft. The intermediate shaft has external splines(teeth) on both ends. Next you have the shaft that holds the u-joint,outer shaft as i'll call it.It has splines on one end and ears for the u-joint on the other
The splined end touches the intermediate shaft, and the eared end stays outside of the axle tube.
There is a collar that has internal splines.
the collar ,when in 2wd rides only on the splines of the outer shaft. When 4wd is engauged the vacuum motor moves the collar over to touch the splines of both the intermediate shaft, and the outer shaft,making the connection to the differential .
the tj axle eliminates the intermediate shaft, and the shift motor and collar.
The tj axle has splines on one end, and ears on the other. It runs from the u-joint all the way to the differential.
YJ's and TJ's don't have selectable hubs, so they are always engauged.
the disconnect on a yj "acts" like a selectable hub, but only with an open (stock)differential. The driveshaft won't spin on a stock yj when in 2wd because of the disconnect.
On a tj there is no disconnect to stop the driveshaft from spinning, so it "acts" like the hubs are always locked, and the driveshaft spins in 2 and 4wd.
I hope this helps, it is kind of hard to understand unless you can get hands on experience to go with the talk
90 MJ short bed converted to 4x4
90 XJ 2 dr 4x4