Your SF 3rd will work fine with the FF and the other way around... IF you have (or get) a cable locker, you need axles with a longer spline. You can get your standard FF axles machined (very $$, I understand) or you can find axles for the locker.
All of the above was such a PITA, that I went with an ARB air locker for a FF axle (RD33, a little stronger than the SF locker). And of course, the advantage of an air locker is you now have onboard air supply.
If you can get the WHOLE cable locker drum to drum, including 3rd, then go for it. Piecing it together is next to impossible, and very very very expensive, even if you can find the parts. I bugged Marv Specter for a year and even he couldn't find any. I called C. Dan to see what was available NEW from Toyota - anywhere - and after about 45 mins on the phone, he concluded nothing was available anymore. There are people who mod an 80 series e-locker into an FJ60 axle, but Dan said there were some possible issues with that should you break an axle off at the splines... Can't remember exactly what he said.
In any case, factoring cost, time, and ease of installation and availability of service parts, the ARB was the way to go for me.