Dig around under the cardboard in front of the passengers feet where the computer is, and there will be a brown box labled "Land rover diagnostic display". When the truck is on, you should be able to see a 2 digit number on there which is the trouble code you need. If it's 44 or 45, those are the O2 sensor codes. Best to do them in pairs if you have to replace them.
Best price for oxygen sensors are Standard Motor Products # SG130, right now rock auto has them listed at $45 each.
If the number is something else, let us know and we can dig further.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.