Do these work under both front and rear wheels, or do I need a pair?

One mat works under a single axle (front or rear). For full vehicle traction on both axles, you'd want two mats. Many buyers grab a pair from the start — they nest together when stored.

How do I keep them from sliding around once I position them?

They're designed to sit flat under the tire footprint. Weight of the vehicle holds them in place once you roll onto them. If you're on extremely loose sand or deep mud, test your placement before committing — adjust as needed before full engagement.

Can I leave these on the ground permanently, or do they need to come out after each recovery?

Pull them out after each use — they're designed for recovery situations, not permanent mounting. Storing them flat when not in use keeps them ready and protects them from UV damage during downtime.