[A. Hudson]

I had no sooner finished reading your opening sentence, when I was pretty sure what was going on. A very good friend used to haul cement out of Marulan and Kandos in the 40's and 50's using Maple Leaf and Chev trucks. He explained to me in the 70's, when I was having the same problem with my C101 4x2. He told me that back then they used to carry a bag of spare bearings and when it got too bad, he could pull the box apart and be back on the road in 30minutes. Now I have never had any reason to doubt his words and he was a very clever man.
At the same time there was a number of 4x4's getting around carrying anything up to 5 ton of water and yet they did not seem to suffer this as the 4x2's. This got the better of me and I started observing propellor shafts and bearings under load. The universal joints on the 4x2 would always twitch when starting off. So where is this going..
I believe that when the propellor shaft exists the gearbox at an angle at the universal joint it is unable to turn with a constant velocity as the angle creates an uneven moment at the joint. The universal is attempting to straighten the rear mainshaft in the gear box, and the only thing stopping it is the pilot bearing between itself and the input shaft. Hence the wear.
On the 4x4 it is not so much of an issue as it travels in nearly a direct line to the transfer case.