This is a textbook case of a disconnected or nonfunctioning TPS, it will screw with power and shifting, and generally create a good case of the shakes, normally you would only be able to generate about 45 MPH, so you're doing OK!!! Normally your idle will be low, and the truck will generally die if you select reverse. It's on the drivers side of the air inlet on the intake, one plug with 3 wires, if I remember right. the lock clips have a habit of snapping off, and they look like they're connected, but they are not. Hope this helps!!!