Is my working area safe?

Hi all,

I'm fairly new to wrenching and I'm looking for feedback on whether my outdoor setup seems safe.

I have a gravel driveway area in my backyard where I want to jack my car up and go underneath safely (i.e. pull a transmission, diff, etc).

In my opinion, I'd say this patch of driveway is 'very flat', however it's still not a perfectly poured concrete garage floor, and is also covered in small gravel.

I jacked up my car and gave it the 'shake test'. I shook it hard as hell. As hard as I could without getting a running start. And everything actually felt really solid. But, I'll admit I noticed very minor flex in things (perhaps because all of it is sitting on wood, on gravel, and on a not-absolutely-perfectly flat patch). Maybe this is normal even on perfect concrete?

At a maximum, the slope in this area doesn't seem to be more than ~2% over any stretch.

I'm using 4x '3 ton' HF steel jack stands to support a 1.5 ton overall car. The jack stands are placed on 12" x 12" x 3/4" pressure treated plywood, on the gravel.

Below is a picture of the driveway area in question, with the car up on the jack stands. Also, I took pictures of my 9" level placed in the middle of the 12" plywood base under each jack stand. I superimposed both directions in each photo.
http://i.imgur.com/9EzPw4L.jpg
http://i.imgur.com/VEjxady.jpg
http://i.imgur.com/rCxGfON.jpg
http://i.imgur.com/LSvAaaa.jpg
http://i.imgur.com/GCAqRm8.jpg

Thoughts on whether this seems safe? My backup plan was to build wooden wheel cribs for each tire. I assume the larger surface area and more solid construction would be safer, hopefully in spite of the imperfect ground. Or maybe there's another recommendation that would work? (Perhaps some style of ramps?)

Thanks!

Since no one else has chimed in with a more "engineered" response, I'll add my thought.

Generally, if you have to ask if something is safe, it's probably not. For example, "Dad, is this carnival ride safe?"

On the flip side though, if I were in your situation and did everything you did, I would tell my wife or kids to check on me every so often, crawl under the car, and get stuff done. I'd just be very careful not to get out the big hammer while working under the car.

-Rod

For extra safety you could put some pieces of wood between the tire and ground or jack it up higher and stick some wheel ramps under tires