1990 Dodge Dakota Overdrive

I have a 1990 Dodge Dakota and Ive had it ever since it had 94,000 miles. I never seemed to notice why I was getting bad gas mileage or why it sounded so loud until I had my dad take a look at it. He figured out that the overdrive isn't kicking in and its over reving itself. Some guy at central auto parts said the most common thing is the overdrive solenoid and that its located on the back of the transmission. We looked and couldnt find it. Can you tell me where the overdrive solenoid is and if this is the problem?