Ok, that' s a relief about the axle diameter. I think I'll just install heavier capacity tires (I don't know what these Carlisle turf tires are rated for, but I can't imagine it's over 1000 lbs.) and run the axles as they are.
Rather than hang the weight from the differential and try to reinforce the axle tubes, I'm going to build my subframe such that it has a heavy transverse member directly beneath the differential that runs out the full width of the thick bosses at the ends of the axle tubes. I'll turn those cast bosses perfectly round and install 2" thick clamp-on collars on them. The ID of the collars will be machined to be a perfect match to the boss, and the collar outer will be square shaped, approx. 4 inches per side. The upper outer ends of the subframe transverse member will be heavily built "c" shackles sized to match those collars. So the whole subframe assembly will slide forward to mate to the tractor, and all the weight will be borne right above the outboard axle bearings. That way very little of the hoe weight is transferred to the axle tubes, barely changed from what they're carrying now.
View attachment 128989
And here it is installed just to give a little overall perspective:
View attachment 128990
I may be able to snuggle the whole assembly just a little forward from where it's pictured now. I'll mount it as far forward as possible.
Not pictured are the outriggers. Around 36" long, angled backward about 15 degrees, as far back as I can angle them and not have them be struck by the boom when it's fully swung to the side.