Axle shaft broke 8 3/4

i know this is a car site but I figured you guys would have a lot of knowledge about the 8 3/4 rear axle and might be able to help me identify why my axle shaft just up and broke in half.
I have a 1991 dodge short bed 4x4 truck that originally had a 9 1/4 in it.
The 9 1/4 was worn out and I decided to throw an 8 3/4 in it that i had in a 74 w100.
It held up great over a year untill today.
I'm driving down a back road going about 20-25 mph when all a sudden the truck starts hopping a little and then I felt what I thought was the transmission slipping.
So I pull over and check transmission fluid level and it's good and the fluid looks good and smells good. I have the truck in 2hi and idling in nuetral and I look down and notice my rear shaft is spinning while the truck is in nuetral.
I thought that was odd, so I lock the hubs in and out the t-case in 4hi and it moves, so I start to pull back on the road and all of a sudden bang! The truck falls on the ground and my drivers side rear tire goes rolling by.
I had to call a tow truck and When I get the truck home I pull both axle shafts and the third member expecting to see some gernaded gears or something.
And when I get the third member off I notice there isn't anything wrong with the gears, so what happened?
The only thing I noticed is the bearing race wasn't on the bearing on the drivers side axle shaft, the one that broke in half.
I don't knowing that happened before it snapped or after.
This is kind of scaring me cause I want to know what happened before I throw it back together cause I don't want that happening on the highway cause that would be very ugly.
Any idea's?