Yesterday I pulled the rear axles out of my '76. I knew it had one trashed bearing and that was evident as soon as I pulled it out. I decided to pull the other and have new bearings & seals pressed on at the same time.
This afternoon, called up the NAPA that was pressing the new bearings for me to check on them. The guy working the shop said he thought the two shafts were different and that he would not be able to press the new bearing.
Does this make sense? I pulled them off the truck that had been rolling on them fine right up until I noticed one of them leaking (I know this is stating the obvious) but what I mean is that there was no apparent problem. Is there a way to identify them to be certain? I don't know much about axles so any help would be appreciated.
The splines seemed to be identical, the diameters seemed close too (by eye).

 

About the only differences would be the type of bearing (ball or roller). The axle/bearing combo will interchange between housings but the bearing is axle specific.
In other words, you can use either style bearing in a given housing as long as you use the axle for that style bearing. The OD of the bearings are the same but the ID i slightly different.
I've seen rears with both style bearings in them.
Look in your axle housing and see if the bearing that was leaking but still okay has a seal inboard of where the bearing seats. If it does that axle has a sealed ball bearing. From the pics you posted it looks like the axle with the bad bearing had a roller bearing with the seal outboard of the bearing.

 

OK SO. I took a little ride over to the pull-a-part down the road and pulled the axle from a 78 2wd. The shaft slid right in and looks like it has nearly new bearings. The gear lube was still honey brown in color. HOWEVER... after I slid it in and bolted it up, I spun the shaft and this is what is happening...
YouTube - new axle shaft making noise.

There is no oil in the dif, but that doesn't sound right to me.

 

Ah hah, I think I figured it out. I believe this is because I got a 28 spline shaft and put it in where a 31 spline goes. So the guy at pull-a-part was not completely accurate in which axles are interchangeable. Anyone know which trucks came with 31 splines?