LSD diff interchange?
 

Will a FC LSD differential s4 fit in my FB s2?

To my knowledge no, the pumpkin's snout is a different length necessitating a custom or modified driveshaft and even then the axle splines and lengths are another possible bunch of head aches. The LSD unit itself is also not possible or else all of us would've been doing it for years by now. Here is a handy guide from mazdatrix on rear diff interchanges of rx7's Differential Interchanges. Also FB LSD unit in decent health is a good unit and the pumpkin is a direct bolt in and my diff that I bought off the forum has held up great behind my cammed V8 over the years. Otherwise if the ZERO aftermarket is a turn off for the stock FB diff and you don't mind doubling the amount of money you have in the car you could buy a complete Ford 8.8 that'll bolt in from here GRANNYSSPEEDSHOP.COM.

Not sure on this exactly, but I did a bunch of research when I made a frankenstein rear end in my B2200, which I think is pretty much the same as a FB large axle rear end.

I am using a Kia Sportage 4.77 ring and pinion, along with a N/A 86 GXL LSD unit in my B2200 housing. I am using the Kia snout, but I put the Kia ring onto the LSD unit, the axles were the same diameter and spline, however, the stock axles rubbed against the spider gear pin inside the differential, where on the LSD unit, there is no spider pin, so I welded on some washers to space out the axles, now they rub against each other, seems to be holding up so far. I had to re-shim my axle hubs to get the correct bearing preload.

When I was looking into this I based my compatibility of the B2200 axle on the large bearing FB axles, if you have a small bearing FB axle, then the LSD wont fit, as I think the splines on the axle where they go into the differential are smaller.