Did you get that from google or something? No offence, but you obviously aren't speaking from experience. Either that or you suck at narrowing rear ends.
With no bearing installed in the end housing you can easily verify if the housing is aligned on-axis to the splines by installing an axle and moving it in a circle. It should not be able to touch the bearing housing at it's extreme limits, and clearance should be the same all around. Assuming the axle is straight, and spline wear is even, this is foolproof.
If the axle is not straight or the spline wear is uneven, then you're screwed anyways.
Getting the end housings square is not rocket science.
You can easily check runout at the housing end with a dial indicator by mounting the axle on a couple of stands with the stands placed near the pumpkin and rotating the pumpkin.
I did all of these checks when I narrowed mine and guess what? It was straight. I actually expected that I'd need to straighten it.
All of the whining and naysaying in this thread is pretty weak. BBQ, bigbegum, and myself have all done this and our diffs check out straight. If you haven't done it or can't do it, don't just say it can't be done.
And suggesting using a domestic ford rear end is an "expensive" way to go about getting a strong rear end is one of the goofier things John V has ever come up with. Obviously you can buy some expensive parts for them, but when stock explorer rear ends are holding 500hp all day long... one should consider reality.
That comes from experience working in a shop that built more than 250 9" rears last year alone for Cup Cars, not to mention those for all sorts of other racecars, including narrowing quite a few stock housings for drag cars. Sure it may not be drastically off, but I'd be willing to bet that its not nearly as 'perfect' as you're proclaiming. It only takes .010" or less to have a profound difference at the contact patch.
I never said it wouldn't work, I just said it wouldn't be quite right. That's your deal though, I'm thinking much more likely you got lucky