just a question, how common are the LSD rear diffs for gto's?
I believe they all have LSDs but you may be referring to the rare Ralliart Hybrid LSDs that were an MR option but were fitted to some std GTOs too. I don't know how many were originally sold, but not a lot. I only know of a handful out there and many of those owners discovered what they had by accident so check your rear diff plate. B4H is the 4WS version and B3H is the 2WS version.
They are still available new from Mitsubishi in Japan but they aren't cheap. I know the cost of buying a B4H and bringing it to the US would end up costing me around $4500 USD. I could buy just the pineapple (same part for 2 & 4WS) for around $3500 USD and swap it into the rear diff of my 3000GT VR4, but that is still too much for me. Still holding out for a used one which still won't be cheap since I will have to swap my trany out for GTO gearing too, since I own a 3000GT instead of GTO.
Hope this answers your question.