At minimum, an engine/transmission skid plate and fuel tank skid plate. These protect the two most expensive and vulnerable components. For moderate trails, add a transfer case skid plate. For serious rock crawling, a full belly pan that covers everything from bumper to bumper provides continuous slide protection over obstacles.

Steel skid plates (3/16"-1/4" thick) are stronger, cheaper ($150-400 each), and ideal for heavy impacts. Aluminum skid plates are 40-50% lighter, do not rust, cost more ($250-600 each), but dent more easily on hard hits. For rock crawling and heavy trail use, steel is the standard. For overlanding builds where weight savings matter, aluminum is worth the premium.

Factory skid plates on Rubicon models provide basic protection with thin stamped steel. Sport and Sahara models typically have only plastic splash guards. Aftermarket skid plates are 2-4x thicker, designed to slide over rocks, and cover more area. For any off-road use beyond maintained fire roads, aftermarket skid plates are a worthwhile upgrade.

Skid plates typically add 0.25-0.5 inches of thickness below the frame, which technically reduces ground clearance by that amount. However, they create a smooth, flat surface that allows the Jeep to slide over obstacles instead of catching on them. The net effect is improved off-road capability despite the minimal clearance reduction.