1G why'd my drivetrain fail?

[Dhan]

So back in January, on my way home from a week in the mountains with family, something broke in my car's drivetrain. It was 2am, on the interstate going 80mph. I felt a very slight shudder from within the car itself. Your average person probably would never have noticed.
The car continued with no diminished power. I smelled a faint burning but it passed quickly. There was no other strange smells or sounds but I knew something wasn't right. Regardless, I drove another 200 miles and got home safe.

I parked it in the driveway, patted the DSM on the head and said "good job", and because it was winter and I have other vehicles, I didn't bother to investigate further.
Fast forward to June when the weather's nice.
Threw the car up on jack stands and began disassembly. When I get to removing the t-case from the trans, it won't budge. They're glued together.
Finally, I realize the output shaft from the trans is loose and can freely move back and forth inside the trans.
So to separate the t-case and trans, I popped off the plate that's on the drivers side of the t-case and slid the ruined output shaft out. A mangled roller bearing also came out.
The output shaft is now seized to the t-cases pinion gear. From there, disassembly went smoothly.

So I'm trying to find out exactly what parts failed and why. I have a replacement trans and t-case in there now, but I don't want all this to happen again.

Is it the viscous coupler that failed? Was it that roller bearing or did that get mangled afterwards? Does anyone have an answer to this scene of destruction?

P.S. all the old parts are sitting in my garage, so if pictures will help to clarify things, I can post some.

 

[donniekak]

It's going to take some pictures to get an idea of what happened. The ball may have been the retaining ball for the output shaft.

 

[pauleyman]

Your tcase was probaby run with low or no fluid at some point in its life. Bearings failed.

 

[Dhan]

Not sure how well these photos will turn out; I'm doing all this on my phone. Wasn't too sure what to take photos of either.

The first photo is the pinion inside the t-case. It still spins freely and smoothly.
Second is the output shaft that goes inside the transmission.
Third is the output shaft/t-case pinion that goes into the t-case. Its seized together where the red dots are. Also in this photo is the failed bearing pieces.
Fourth is looking down transmission. Its the opening where the output shaft goes.

The car had 155k when I got it in 2012. The guy said the trans, t-case, and rear diff fluids had been done recently so I just trusted him. The drivetrain was worn though. Grinding synchros, poor clutch disengagement, and the inside of the bell housing is caked with oil and filth. Overall, the car's previous 6 owners haven't been kind to it.