1998 Dodge Ram Speed limit

My van will not exceed 65 mph, it's 5.2 liter engine. We've changes plugs, wires, belts, oil and all filters also we just had a transmission tune up. However, she still won't exceed 65 mph. I bought the van used and we're planning to go on a trip with our grandchildren in the near further. Does this car have a some kind of limit switch if so how do we get it fixed. Thank you for your help in this matter