As stated before, if they want it, they will get it. Drop the driveshaft and tow it, or trailer it. Disabling the ignition won't matter in this case. To make it more difficult for them to do that, park next to a tree if possible and run a chain around the tree and the rear axle, padlock it. Also, you can get a manual line lock (the kind the early 80's international trucks with hydraulic brakes had) and use that to lock the brakes down, electric line locks are only good for about 2 minutes at best, and they draw power, when the battery dies, it releases (providing it makes it past the 2 minute mark). About the only other thing you can do is to run a chain through your rims to the frame. This will not prevent them from taking it, but it will slow them down/make them think twice about stealing it...
SVT