changed j1400's final drive from a 3.7 to a 4.18 so thought i'd do a thread on how its done, maybe it could go in the archives or something?
firstly, to check what final drive you have, look for this number, under a few layers of dirt, on the side and you'll see the f/d number there, yes it does exist!
now, get the relevant final drive that you want, and yes you need both pinion and crown wheel
now, start by removing the turret
now remove all the thirteen mm headed bolts from the sandwhich plate, now you can lever the cluster out of the box
now remove the final drive pinion, by firstly removing the circlip and either levering or pulling of
at this point we had a bit of a mince
now you're ready for the diff removal, remove the diff plate, then undo the bolt on the retaining tag for the bearing carrier castellated nut, and mark its position so you get the bearing pre load right on reasssembly, now, using a flat bar or similar, undo the nut and remove, then remove the diff assembly
undo the nuts on the pinion and remove it form the diff, this may need a tap or two, for which you can use the box as a support!
now fit your new one, and do the bolts up to torque, thenease it back into the box
then refit the cluster, remembering to fit the thrust washer back on the reverse shaft, it only goes on one shaft, so its pretty obvious where it goes, use new gaskets, and do the bolts back up to torque and you're done!
so there you go, hopes this helps anyone thinking of tackling thsi job but thought it was too scary
*if you are using new beaings, you cannot simply do the casstleated nut up to where it was before, you must check the meshing of the pinion and crownwheel by the use of engineering blue on the teeth as desribed in the haynes guide*