Admittedly this is right off the top of my head, and I haven't really thought it through, but here's a possible solution to hoarding:
After a jeep is found, and the finder enters the contest, their entry is NOT valid until the same jeep is found again and the contest entered by the next person. Some reasonable time frame should also be applied, say maybe 30 days tops. So if you find one, enter, and hang on to it for 31 days, your entry is void, though the person who found it before you is OK, since it was found again.
Thoughts, anyone?