Whatever you do, chock the wheels with proper chocks or at least wood 4x4s with a slight slant cut into them. The last thing anyone on here wants to see or hear is that someone was crushed by their vehicle because it rolled while they were under it.
I always chock my wheels when I go underneath my truck, and it saved my life a month or so ago. My 01 has all steel linkages for the shift lever, and one of the retaining pins broke or fell out and it fell apart and got stuck in park. I chocked the wheels and tried to get it back together without taking it out of park, but it just wouldn't reach. I was parked on flat ground but didn't notice my front tires were partially up on the curb. I pulled it out of park and the truck lurched backwards and pushed the chock about a foot backwards bit in and brought the truck to a halt. I tried to roll under the middle of the truck to get out of the way of the tire, but my shoulder got stuck on the frame rail. When the truck stopped I looked up at the tire and it was inches from crushing my head.
If I hadn't chocked my wheels that day I would've likely been killed despite my truck being off and on flat ground.