The stock nut isn't welded in; it sits inside a metal box which is welded in. I suppose there may have been some revision or early version where the nut wasn't free, but none of those I've worked on have been welded directly in. Putting the nut free in a piece of U channel and tacking that down, similar to the stock arrangement, might not be too difficult.
A break in the place you're talking about isn't a power thing... it's a torque and excess play thing. Your diff's been pounding on it like a slide hammer... this could have happened in a Honda. Time to start going over the car and torquing all the critical fasteners... start with the subframe fasteners next.
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