Questions about 1992

MY first question is....Is it normal for my truck to have a high idle then when pushing clutch in it will drop down to normal idle speed? Any solutions to this...Also i have very little heat I dont have any leaking in my passenger floor and i changed the thermostat so what should i look at next? Finally i am getting very poor gas mileage about 9 miles per gal. I think theres a possibility its stuck in low range =...when startting the car all the lights come on ( kinda like a flash but dont stay on) except the 4 x 4 and low range.