Where is the thermosstat located on 1998 ford ranger xlt 4.0 engine?

The thermostat on a 1998 Ford Ranger 4.0L engine is located on the engine block, at the lower passenger side of the engine, near the water pump. It's usually a fairly prominent housing, often metallic, that the upper radiator hose attaches to. You'll likely need to consult a repair manual or a diagram specific to that engine for a precise visual location, as access can vary slightly depending on accessories and other components.