Aha, now you're onto it! If you can just get that spline out for a cleaning, polishing and lubing then this should be totally repairable. A wire rifle brush (thin bottle brush) would be useful for cleaning the bore of the bushing.

Multiple applications of a good penetrating oil like PB Blaster over a few days may get through enough of the corrosion to be able to turn the spline back and forth a bit and slowly work it out. Maybe apply a bit of heat a few times to help break the bond and also wick in. I doubt there's anything inside to melt and burn other than trace lubricant, maybe a slim chance of a fibre washer on the back end of the spline.

Failing that, my next step would be thinking about trying to dissolve some of the aluminum corrosion that may well be behind this, such as at the back of the aluminum casting. In order to avoid damage to the exterior surface and threads, if you can lay your hands on what's called aluminum jelly (an acidic form of petroleum jelly) and put a small bead around both ends of the spline at the casting then that may help dissolve it enough to get it moving. You may have to leave it for quite some time and again apply heat to get it to wick in. Start by using a spray solvent, like contact cleaner, to remove traces of the penetrating oil.

I'm looking forward to your success as this could easily happen to me as both of my 940 wagons get highway use in the wet. The back ends are always getting coated in road spray with dirt and even salt. Mind you, I'm always using them so hopefully they don't get a chance to seize up like yours.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now