RE: fs racing diff
if you follow fs*mt*4wd's steps in his diff mod thread your diffs will be as smooth and as reliable as the mcd's.yes the mcd diffs will fit in the front and rear, but they are the same type of diff and are prone to breaking the same way the fs diffs do. the new diffs(evo3) are better but are also big bucks.
when i did my diffs i spent alot of time filing my gears, not only on the unsquare ends but everywhere. it sounds like you may have a bent pin in you diff. i would look into fixing your fs diffs before i spent that much on mcd diffs that have the same design.
as for the center diff im pretty sure it will work but i am not 100% on it. i am suprised that no one answered this for you sooner.