If you go under it, just bolt the tires back on if possible. Then if it does fall, the is some clearance between you and the truck. I put mine under each axle as wide as possible. If working on a axle, I put them just behind the spring hanger on the frame, usually setting on solid concrete blocks, DON'T USE HOLLOW BUILDING BLOCKS! My jack stands won't go high enough to get the axles off the ground with out setting on something.
Always rock it around for good measure.
Joe
