Unless it was special ordered, none of the MN12s/FN10s came with 3.55s from the factory.
The options now are to pick up an open diff and swap the gears out.
New gears aren't terribly much - FRPP gears can be bought for $150 from reputable dealers out of MM&FF magazine (Tommy Vaughn has a nice listing in every issue I've gotten over the past year.)
Some of these places will also have the rebuild bearing / seal kits.
RwP
Thanks for the quick response, so any 89-95 posi or open diff case should work? can you use the stock stubs? they are 27 spline right?
I am planning to run high torque, high hp, whats the weak link here, the case, the stubs, or the half-shafts?
Those mentioned models all have bolt on half-shafts?
For that, if you stay with the IRS, I'd HIGHLY recommend (and if Dalke or one of the other pros chimes in with other suggestions, ignore these recommendations!) that you:
a) Go auto.
b) Use DLF's Delrin bushings on the LCA and spindles both.
c) Upgrade to Cobra 31 spline half shafts and a Cobra 31 spline rear end.
d) Use something besides a tracloc - one of the Detroit family, possibly. Or even a spool instead of a tracloc design.
e) Buy up all the Mark VIII half shafts you can find and have them resplined to run 31 spline
f) Hold on and pray every time you launch
RwP