Were the diff & shafts from the same car or were they pehaps obtained from sepperate sources ?
It may be possible that you may have a set of push in shafts & a diff that had the lobro type shafts fitted ( Someone may have removed the stubs from a lobro diff )
As posted above, it is well worth checking the dimensions of the shafts & diff input bearings with a vernier caliper.
It only takes a very small burr or tiny bit of rust to stop them engaging
Keep us posted on your progress