I suppose you will put in the transfer box as well, that will give you permanent FWD. The Series front axle cannot deal with that because it has UJ's not the CV joints of Defenders. It's possible of course but you will have to change most of the drivetrain. plus I think the gearbox mounts are different.
Plus any axle change you've got the suspension to deal with as any later axle is set up for springs not leafers.