The 6x6 front diff IS a "Rover" diff. The housing is cast instead of the fabricated design usually seen, and the centre has 4 pinions instead of the usual 2 pinion, but these are just "beef up" options for the Rover unit. In all other respects it is a Rover diff.
If you want to change the front diff ratio to 3.54 and keep it the same design as now, you will have to source a 4 pinion 3.54 centre from a 4x4 RFSV or Survey to get the right offset for the 3.54 gears (but a 2 pinion centre from a 4x4, or Disco, RRC, etc will fit too) . Failing that, any of the replacement centres for traction aids for a Rover diff will work, ie Ashcroft ATB or Air Locker, Detroit Truetrac, ARB Air Locker.