Aye, CAPT Kirk is exactly right.
When completely assembled, the axle shafts are the motion limiting devices on how far down the wheels can go from the body of the car. During the maintenance action of removing the axles, the manual tells you to also take the brake calipers off the rear wheels and tie them up to the body. The reason is, once the motion limit of the axles is removed, the next motion limit is the flexible brake lines!
During disassembly, you often have to jack up the back of the car to get the wheels off the ground, and further loosen the differential bolts and jack up the differential a bit further. That gives you the clearance to get the axles out of the hubs. Assembly is the reverse of course. Let the differential back down if you had to jack it up, and raise the wheels relative to the body of the car. You can do this by lowering the car (but hopefully you are on jackstands, not a jack) or putting a jack under a wheel hub to push it up.
Just remember the next time you take your W123 airborne over the top of a hill - the only thing holding your axles in are those two little bolts in the hub ends.
Ken300D