I am not in favor of the axle saver bearing because it doesn't in fact save the axle. They move the bearing inboard but the axle isn't hardened at that location so it will wear through even faster since it isn't as hard and you have twice the weight on a smaller area. If the axle is worn out it needs to be replaced and new bearings and seals used.
That being said I still suspect the posi as sticking. Modern axle fluids are not as good as the older stuff (which used to be made of whale oil), but as you might expect in todays tree hugging environment killing whales for the light oil in their blubber is out of the question.
I recommend Texaco 9622; Mineral based 80W90; which is the same as GM Part # 9985290. Do not use synthetic gear oil with their slippery silicon based additives.