the 2wd diff is wider at the top than the 4wd diff which is why it won't actually fit into the beam, but it's not structural, only for the bolt to go through so you asn remove it without any problems
the other major differences are that the 2wd diff has 108mm output flanges and the 4wd one has 100mm output flanges
and you can't just get away with changing the shafts for 4wd ones either because the other ends where they gfo into the stub axles/hubs etc are also bigger so these would need to be changed as well

a bit of an arse reallty