Series 2 and 3 Transfer box High Range Conversion done by Ian Aschcroft Conversions UK

The mod happens as follows.

In high range mode your transfer box has 3 gears in circuit.
1. The gear that sits on the end of your normal 4 speed box.
2. A large intermediate gear that sits in the middle.
3 .The gear that spins on the transfer box out put shaft.

Now, 
a. Gear 1 has it's spines machined out so that it can spin in gear 3's position.
b. Gear 3 has splines welded in so that it can be bolted back in gear 1's position.
c. The intermediate gear shaft holes are filled up and re-machined to suit the new gear position.
(I don't know if the original intermediate gear is used )

End result------plus minus 25-30 % more top speed fuel saving but with the same low range as normal.
Problem is that the gears are now unique and must be made up when required.
-- Bruce Molzen

WARNING:  This is some other advice we stumbled on:

Don't touch the Xfer box 'upgrades'. If you're not looking at an entirely new casing specially built for the upgrade, the only way anyone can accomplish the change in gearing is to move the pivot point of the gears to an unsupported location, which creates a weak point.
-- Paul Oxley

This will be subject to the support webbing within the transfer box. Use your discretion when considering this option as the costs involved in the gear change are unlikely to be less than swapping your crown wheel and pinion sets for 3.54:1 ones, though this would affect your low range as well.
-- Landyonline

LT 230 Transfer Boxes

These transfer boxes are found behind the LT85 / 77 and R380 Manual boxes and behind the Discovery 4 speed ZF Automatic.

There are 4 different gear ratios available for these boxes, they are all interchangable.

1: 1.667 Originally fitted to the 4 Cylinder 110 and possibly the 2.8i Defenders

1: 1.42 Originally fitted to all V8 110 and 110 Defenders as well as 4 cyl 110 TD

1: 1.193 Originally fitted to All Range Rovers and Discoverys

1: 1.003 Special gear set for conversions, supplied by Land Rover Engine and Gearbox Specialists - UK (L.E.G.S)