An old Jeep CJ with a hard top is a good choice. Pretty reliable, carbonated engine, capable offroad, pretty common. Toyota 4runners are also great. If you've got the money for it then Toyota Landcruisers or H1 hummers are good. Diesel surplus military pickups are available for next to nothing and are pretty tough

Infact any surplus military vehicle is typically fairly tough (b/c they have to be).