I would immediately check your coolant level as well. If it doesnt read at all, sometimes that means there isnt any coolant to get a reading from!!! This being said, get the thermostat replaced. It could also be an electronic failure sending the signal to the gauge. I agree that you need to check your fluids. While youre at it, check the tranny and transfer case. Only takes 2 or 3 extra minutes. Also, try lubing your rear driveshaft as it is known to provide the "klunk" going in and out of drive and reverse. This way you can troubleshoot to lesser numbers of things.