If you are going to keep the combat rims so it looks right, you would be shooting yourself in the foot to use anything but the correct tyre. They are available, even over E-bay at $140 including delivery:
http://cgi.ebay.com.au/New-Tyre-4x4-...item2eb1ace110
Maybe you need a keener tyre man? All the Jeep guys over here seem to be able to get them, I would bet that Marathon Spares have a container load as well.
Problem as I see it is using anything taller will affect your gearing, and you only have a 3 speed box and a standard engine.
Thats my 2 cents worth....
Rich.