1991 Ranger 4x4, 4.0 engine. On the drivers side below the power steering pump and toward the frame there is a small shock that mounts to a bracket attached to the engine. This is probably the original part. What does it do and how critical is it to replace?

 

That is called an engine idle damper. Only on early 4.0L engines for a few years until the idle roughness was improved internal to the engine. Don't know how effective it is after 20 years. Try disconnecting it and see if your idle changes much.