Hi Paddy
Welcome to the forum, you'll find it a FONT of knowledge for everything Patrol!
Search IS your friend!
In the case of transfer case gear, the high range stays the same. The two main (only?) brands of gears are Rockhopper (80 odd% reduction) or Marks gears (which come in 23%, 43% and 80 odd%).
I had the 80-something% Marks gears in a Mav with 4.6 diffs and 35's and they were AWESOME! The only better mod on the car was the air lockers!
You need to think about exactly WHAT you're going to use the car for to decide the ratio. The 80% gears mean that you'll be switching from hi to lo ALOT, the 43% seem like a good compromise (if I put any in the GU, I'd put these in) the 23% gears seems hardly worth it, but, horses for courses! Good on them for offering them, not EVERYONE goes rockcrawling every weekend hey!
Cost wise they're about $1200 to buy and probably $500-800 to fit. You could obviously reduce that by taking out the transfer case yourself.
With 285's I didn't think there would be THAT much difference to the std 265's, but the Patrol's 2.02:1 std low range is on the very end of acceptable. Even the 23% would gears would make it ok even with bigger tyres!
Just google 'rockhopper gears' or 'mark's gears'.
Good luck with it!!