That makes sense. I've read about the temp guage issues, just wasn't sure if that affected the odo. I found another that was suspicious. Talked to the dealer. Said there weren't any records and was sending me autocheck and carfax. 2003 with 19k miles.Yes there were a large number of Gen III's that had bad clusters that affected the temp gauge (and possibly the fuel gauge). The typical fix was replacing the whole cluster, which reset the Odo, but it can be fixed by reflowing some traces on the cluster ECM.
I recommend any Gen III Montero buyer to get a carfax and personally I would be skeptical of any low mileage examples.
You should be good as long as the Carfax "story" make sense. If you can take a look at the truck, it should be in pristine condition with such few miles. Little things that are worn are a dead give away (like dash bulbs burned out, excessive seat wear).
I doubt it will show anything different, but you can use Ebay's AutoCheck as well (there is a trick to get it for free).
Low mileage trucks are out there as I just picked up my '06 with 56k on the odo. There was a black and tan one in NC with ~40k that I missed by one day a couple of months ago.
Good luck and great catch!