Yukon gets their gears from multiple lower tier manufacturers. The splines aren't cut right. Take a wire wheel to the splines and see if you can clean them up
They don't seem rough, it's like they're the wrong size or something??
I can't tell if they're actually different from the old pinion. They look the same side by side but who knows. New is on top.
I suppose perhaps the way I modified the old pinion nut to be a setup nut (no longer a lock nut) weakened it to the point it starts falling apart before the yoke goes on far enough? I was planning on trying to get a normal hex nut in the right size to see if that is what's going on.
Ground off the top conical lock section, and then drum sanded the threads. I didn't really want to grind the threads down this far, but it took a lot to get it to spin freely on the new pinion threads, whatever that means. I got it to spin freely on the old pinion, and then it took 4-5 more rounds of the dremel drum sander before it did the same on the new pinion.