I don't think there is anything 'modified' transfer box, no one else has heard anything of it.
Bristol Transmissions had some advertised much cheaper than teh Vauxhall prices being bandied around:
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http://www.bristoltransmissions.co.uk/shop/home/155-antara-22-transfer-box.html
GM 23247712 Transfer box for 2.2 Diesel with Auto Transmission
Reference: BTL3090 Condition: New product Vauxhall Antara
transfer box
Fits automatic. (GM6T40/50) OE Number GM 23247712 £2,240 tax inc.
http://www.bristoltransmissions.co.uk/shop/transfer-boxes/115-vauxhall-antara-22-transfer-box.html
GM 23247713 Transfer box for 2.2 Diesel with Manual
Transmission
Reference: BTL3091 Condition: New product Vauxhall Antara
2.2 Transfer Box/Case
Fits Manual transmission only. £1,560 tax inc.
Warning: Last items in stock! Carriage quoted only for UK customers
Please ensure tyres are matched, correct size and pressures.
Worn or incorrect tyre sizes will cause premature failure of transfer box and
invalidate any warranty