So at this point, we are in agreement that the one piece rear drive shaft is your problem.
Putting a high pinion front diff from a FJ80 in the rear of your truck would be a great idea if it wasn't weaker when run backwards.
With a standard rotation ring and pinion, when you rotate the pinion to go forward, by design, the pinion is being pulled into the ring gear. However, when you shift into reverse, it is much weaker because the pinion, by design, is being pushed away from the ring gear.
The high pinion when mounted in a front end of a truck is stronger when going forward but weaker when backing up. This would be reversed when installed in the rear of a truck. By design, going forward would push the pinion away from the ring gear making it weaker than your current rear diff, but driving in reverse would pull the pinion into the ring gear making it stronger than your rear diff.
You have a great idea, but the 8" ring gear is too small for the task. Even Dana 44 high pinions fail. The only reason why Dana 60's high pinions live is because of there brute size and strength.
