aww, come on now, the 4x4's are safe. He does, after all, have the rear wheel chocked with some kind of brick...

 

Looks pretty safe to me - I've seen more precarious building structures that are hundreds of years old.

Haven't done the numbers but seat of the pants engineers guess says that a spare D90 wheel will be way strong enough to hold up the raised corner so long as the spare is supporting a wheel with a tyre on it.

I'l think the best solution in this case would be a stack of pieces of 12" x 2" - these would spread the load and be stable enough.

Jackstands are out on dirt and you will probably find you need to put the jack on a piece of 10x2 to prevent it digging into the dirt.

 

Nothing beats finding a concrete pad. Somebody has one you can use. But if you must....

Don't use a wheel. Don't ever use jackstands on dirt. If you can't find a natural curb area to get under, this type of ramp is your best option. They are useful on concrete too. If you take some time in the design phase, you will find you can drive onto this type of ramp without removing your front spoiler (if applicable) Be sure the dirt you are on is firm. Chock the wheels. It is up to you to decide what is safe. BE SAFE.

 

I use railroad ties. Never a problem.