So the trick seemed to be just spraying the splines with white lithium grease o_O
That at least got it to slide in 1/2" and I thought I had it until I looked closer and realize it wasn't inside the knuckle yet.
Knowing the splines were definitely and heavily engaged, I didn't mind giving it light taps with a dead blow hammer.
Wonder if it was friction on the splines still and I needed even more lube? I could turn the axle so I don't think it was corrosion on the knuckle holding it up (I cleaned that up the first time I worked on it years ago -- couldn't even pull out the axle for corrosion blocking the u-joint).
Next up is the the diff -- order a 5/16-18 helicoil kit. 5/16-18 is listed as the factory bolt size for the JK Rubicon. 5/16 being 7.94mm which is why it looked metric when I was gauging it.