1989 Ford Ranger 4 Cyl

Hi All i found a 1989 ford ranger for sale here in town for 800 bucks its not a bad looking truck interior is great 235,000K on the motor runs great. my question(s) are #1. Why is the check engine light always on?? there doesnt seem to be anything wrong with that motor to me. #2 how come it shakes like crazy at about 90km/h (approx 55 mph)? it does have winter tires on the front but would that cause it??? thanks for your help. Also one more thing i forgot just editing the post The Temp guage always stays on Cold even after driving it for awhile im thinking that it needs a thermostat could that be why the check engine light is on? and is this a serious matter?