It’s obviously not good, but I don’t think it’s the end of the world. I’ve seen gear sets with rust pitting like that, that have still done lots of miles. It all comes down to what you’re planning on doing with the truck. If you’re using the truck every day for work hauling around 15k lbs of trailer, or dragging a 5th wheel RV across the country, I would recommend you fix it right, right now. But, if you’re driving 10k miles per year, mostly empty, or pulling only a couple thousand lbs, it’s not so critical. Assuming you’re in the latter camp, I would clean it up the best you can, put fresh oil in it and run it. In 500-1000 miles drain the gear oil again, and look closely for signs of metal. If it’s not too bad at that point, you’re probably going to be ok for quite a while. Maybe change the gear oil at every other oil change, just to keep an eye on things. The thing to keep in mind is that if things go bad, the most cost effective fix is probably going to be swapping out the whole axle with a good used one.