If you get too much wheel-spin while climbing a slick clay hill, and the truck shifts it's weight and the spinning tire suddenly has LOTS of traction, the D44 axle-shafts can go "Bang".
That's how both Mark and I blew out our axles (the two friends Jason mentioned). I've got the open, and the truck just shifted a bit, and the spinning wheel got traction, and that ended the shaft.
Mark slide sideways into a berm with his, and took out the joint, and the ears came around and tagged each other.
My breakage was mostly stupidity, I knew I wasn't going to get up the hill...
Mark should have been fine, but his joints were starting to go, and they couldn't handle the sudden load.