I think I started the thread OP is referring to. I took mine to a heavy truck shop, they put in two crossmembers, a 10k hitch, and re-did the frame extension welds. Said I should be good to tow up to the chassis limits structurally, but that rear axle weight or possible front/rear balance would become the limiting factors and to make sure I visited a scale with whatever I was intending to pull.
It's hard to get a definitive answer on the legality, but if I'm within the GCVW, axle weight, and receiver hitch limitations I think someone would have a difficult time making the case that I was negligent in any way. Vehicles get modified all the time for various uses.
