Help with wheels and tyres!

I'm really struggling to find an off road wheel and tyre combination for my '99 Series 2. I've got 18 inch alloys on at the moment with on road wrangler tyres. I need something more suitable to occassional off road use - I'm happy to take them off and put the wranglers back on for on road use i.e. I dont expect to keep the off road wheels and tyres on for any great length of time. Can anyone help?

Cheers

Paul