Why my Jeep won't gear down when going down a hill. The tach races up to 3-4

Hello, I would first recommend seeing your local Jeep dealership to see if what you are experiencing is "normal operation" or evidence of a problem. Assuming you have under 36,000 miles at this point, any concerns would be covered under factory warranty unless there's some kind of physical damage or intervention. I would not expect a transmission to downshift going down hill arbitrarily. The tachometer, however, does seem too indicate a downshift might be happening if the engine speed increases.