does it have a solid diff? yes ... so it's probably not IRS.
<end patronising voice>
as for converting to disk brakes - you'd need to either:
find a similiar width axle, adapt the mounts to suit, plumb up
or
find suitable end-plates from disk-brake axle, adapt to MX62 axle
i think wilbo has done research on this and he probably has a short answer?
however, you could also search the forums for MX62 cresidda and disk brake conversion threads?